Hydrogen (H 2 ) is produced in the rumen during the microbial fermentation of dietary carbohydrates, and is consumed as an energy source by H 2 -using microbes, especially the methane (CH 4 )-forming methanogens. In vitro fermentation systems are used to study some aspects of rumen activity, and the kinetics of H 2 accumulation in these systems is a balance between H 2 production and consumption. The fate of H 2 produced during fermentation is either as dissolved H 2 in the liquid phase (dH 2 ) or H 2 gas (gH 2 ). This study proposes a mathematical model of the processes leading to gH 2 accumulation. The gH 2 was mathematically divided into a dissolvable (x) and non-dissolvable (y) gH 2 fractions. The gH 2 in fraction x was assumed to be available to re-dissolve into the liquid phase and therefore could be consumed by H 2 -using microbes. The gH 2 in fraction y represents gH 2 that does not re-dissolve into the dH 2 pool, and thus biologically unavailable for the H 2 -using microbes. Our model was developed to describe the changes in gH 2 in an in vitro fermentation system, based on gH 2 production and re-solution. Seven very different profiles of in vitro gH 2 curves were selected to demonstrate the applicability of the new model, including gH 2 accumulation profiles from two feeds and three methanogen inhibitors. The three inhibitors reduced CH 4 production in different ways: by partial inhibition of methanogens (bromoethane sulphonate), by complete inhibition of methanogens (root of Rheum palmatum), or by acting as a hydrogen sink (nitrate added as NH 4 NO 3 ). The new model fitted all seven curves of in vitro gH 2 kinetics satisfactorily. The fitted parameters varied between the different in vitro gH 2 curves, therefore allowing description and classification of the curves, and the underlying interpretations were consistent with current knowledge of H 2 production and consumption. In summary, the mathematical model described here provides biologically meaningful parameters to interpret the process of in vitro gH 2 accumulation. Use of this model in future studies with in vitro systems could confirm its wider application to describe gH 2 kinetics when different inhibitors of methane formation are applied.