The U.S. Court of Appeals for the Federal Circuit has undone a win for renewable energy firm Gevo and its lawyers at Cooley in a patent showdown with Butamax, a biofuels joint venture formed by BP and DuPont.

U.S. District Judge Sue Robinson of Delaware had granted Gevo’s motions for summary judgment for noninfringement and invalidity for lack of a written description against Butamax in March 2013. Robinson also denied Butamax’s bid for a finding of infringement.

But in a highly technical ruling authored by Circuit Judge Richard Linn and joined by Chief Judge Randall Rader and Judge Evan Wallach, the appeals court reversed in part, vacated in part and remanded.

“The appeals court offered a new interpretation of a disputed claim term, vacated the district court’s prior rulings, and ordered the court to reconsider issues related to infringement and invalidity,” Colorado-based Gevo stated in a press release.

The two patents in question were developed by DuPont and cover the use of a particular cell in a method to produce the renewable fuel additive isobutanol. They are part of a patent war between the two companies that began in early 2011 and has spiraled into more than a dozen separate suits.

Butamax has been represented by Kirkland & Ellis. New York IP partner Leora Ben-Ami argued the case in the Federal Circuit. Cooley partner Michelle Rhyu argued for Gevo.

The dispute focused on how Butamax defined a claim term in the patents and whether that definition was different from its plain meaning. For a patentee to change the plain meaning of a claim term it must clearly “express an intent” to redefine the term. Robinson analyzed Butamax’s definition of the catalyst known as KARI in its patent and sided with Gevo that the term had been defined more narrowly than its plain meaning.

The Federal Circuit’s ruling finding otherwise likely broadens the scope of the patent. The court remanded the infringement question back to the district court based on the plain meaning of KARI and awarded litigation costs to Butamax.