G1/23 in practice: The Board’s Preliminary Opinion in T 438/19

It’s certainly been a busy summer for the EPO’s Enlarged Board of Appeal (EBA). Since the landmark decision in G 1/23, and our deep dive into the decision, we’ve been keeping a close eye on how the Boards of Appeal are applying G 1/23 in practice. Last week we saw the referring Board 3.3.03 issue their preliminary opinion in the case which triggered the original G 1/23 referral, T 438/19. This case offers us our first insights into the practical implications of the Enlarged Board’s decision and how the Boards of Appeal will apply it.

Background to T 439/19

As a refresher, the appeal began as an opposition against EP2626911 owned by Mitsui Chemicals. The patent relates to a polymer sealing material for solar cells and had been opposed by Borealis AG. During the Opposition proceedings, the Opponent alleged that a commercially available polymer product (ENGAGE® 8400), which had been put on the market before the priority date, was the closest prior art for inventive step. However, the patentee argued that while ENGAGE® 8400 was indeed put on the market, it was not prior art because it’s method of manufacture was not known, and the internal composition of ENGAGE® 8400 was not readily reproducible, in line with an earlier Enlarged Board Decision, G1/92.

The Board decided that this question was decisive for the appeal and referred three questions to the Enlarged Board of Appeal (EBA) as G 1/23. The EBA handed down its decision earlier this summer, and Board 3.3.03 has now resumed hearing the original appeal. Oral proceedings in the appeal are scheduled for next week, 15^th to 17^th October 2025.

For more information on the background to the referral, make sure you see our previous article.

Can an unreproducible product be the closest prior art?

Before the referral, the patentee had argued that ENGAGE® 8400 was not prior art at all, because its composition was secret and not reproducible. In view of G 1/23, that argument is now untenable, as the Enlarged Board was explicit in overturning the requirement of reproducibility which had been set out in G 1/92. The question now becomes how this prior art is relevant for inventive step.

In G 1/23, the Enlarged Board explained that all analysable properties of a product put on the market become part of the state of the art, even if the skilled person could not reproduce the product on their own. However, the decision also explained that “the fact that the skilled person would possibly recognise that they are unable to reproduce some important property of the product may also represent relevant technical information” (G 1/23 para. 92 of reasons). As such, G 1/23 suggested that reproducibility of the prior art would still be a factor when considering inventive step.

In their latest submissions, the patentee has fully embraced this position. They argue more emphatically that a commercial product that cannot be reproduced cannot serve as a realistic starting point for the applying the problem–solution approach. Since the skilled person would first need to establish how to prepare ENGAGE 8400 before modifying it, and changing the catalytic conditions would not be trivial, they would instead start from a polymer whose synthesis is disclosed in the prior art. That, they argue, would be a more promising starting point than a polymer that cannot be easily reproduced.

Key points from the Preliminary Opinion (2^nd October 2025)

As is the usual practice for the Boards of Appeal, the Board have issued their preliminary opinion, which sets out in advance of the oral proceedings, what the Board sees as the main issues for deciding the appeal. The Board’s initial opinion therefore reveals how receptive the Board is to the patentee’s arguments.

Firstly, the Board consider that the ENGAGE® 8400 polymer is prior art under Article 54(2) EPC. This is unsurprising as it follows directly from G 1/23. The Board also confirm that it follows that the technical information in other documents (e.g. density, aluminium content etc.) concerning the ENGAGE® polymer is also disclosed.

More interestingly, the Board appear to consider that the closest prior art need not be fully reproducible. In the Board’s opinion there is a sharp distinction between the selection of the starting point and modifying the starting point, suggesting that the patentee has conflated the two different stages of reasoning.

In particular, the Board stressed that “What needs to be modified is part of the inventive thinking of the skilled person in order to solve the problem addressed, but not a consideration concerning the selection of that starting point.” In other words, the challenge of altering a partially reproducible commercial product is itself the part of the inventive step analysis but not a reason to reject it as the starting point. It follows that the Board are currently of the opinion that the disclosure of the ENGAGE® 8400 is a suitable starting point for inventive step analysis.

Looking ahead

Overall, the Board’s preliminary view suggests a measured approach to interpreting G 1/23 and confirms that products which are readily available in the market are effective as prior art disclosures for both novelty and inventive step and can no longer be disregarded on the grounds that they cannot be reproduced by the skilled person.

The Board also appear to be unconvinced by the argument that an unreproducible product cannot be the closest prior art. If the Board maintains this view, it will have the effect of keeping the EPO’s inventive step analysis as a test of technical obviousness, rather than solely being an assessment of the ease of reverse engineering, at least in the case of complex products such as polymers.

Finally, it is important to stress that this is only a preliminary and non-binding view ahead of the oral hearing. Although the Board will give their decision orally at the end of the hearing, their reasoning will follow in a written decision to be issued afterwards, and we’ll be awaiting that with interest.

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