Availability analysis is applied to the cylinder of a spark ignition engine during the closed part of the engine cycle when biogas-hydrogen blends, with volumetric fractions of hydrogen up to 15%, are used as fuel. The focal point is on the demonstration of the spatial distribution inside the burned gas of the combustion-generated irreversibilities for the various hydrogen concentration cases examined, which constitute one of the major sources for the defective exploitation of fuel into useful mechanical work that cannot be identified by the traditional first-law analysis. For this reason, an experimentally validated closed cycle simulation code is used, based on a multi-zone thermodynamic model of the cylinder content, applied in conjunction with a quasi-dimensional combustion model for burn rate predictions. After presenting global availability-balance-related results, pointing out the increase in the second-law efficiency of engine operation with the hydrogen enrichment of biogas, detailed information is provided regarding the spatial development of the combustion irreversibilities throughout the thermodynamically inhomogeneous burned gas, along with their link with the developed temperature field, as determined during combustion at each hydrogen fraction. It is revealed that the addition of increasing amounts of hydrogen in biogas promotes the degree of reversibility of the burning process mainly during the combustion of the later burning gas, due to the incurred increase in its combustion temperatures. On the contrary, the contribution of the early burning gas to the decrease in combustion irreversibilities with hydrogen addition seems to be less prominent. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.