In this study, the gas formation of anaerobic digestion was analyzed by the shrinking core model. This model is based on the mass transport equations. The experiments were carried out with hydrothermal treated wheat straw. Additionally a control group of untreated wheat straw was examined. With untreated straw the beginning of microbiological growth was limited by convection through the surrounding fluid film. With further incubation time the bacteria formed a biofilm. Diffusion through this layer limited the degradation. A short hydrothermal treatment decreased the convection-limited phase. The gas yield of the straw was 0.54 dm(3) (0 degrees C, 1 atm) per gram volatile solid. The pretreated straw yielded in 0.51 dm(3) (0 degrees C, 1 atm) per gram volatile solids with the same mean content of methane (49 vol%) and carbon dioxide (51 vol%). (C) 2013 Elsevier B.V. All rights reserved.