BACKGROUND: Sodium ion, a common constituent of food, is empirically observed to inhibit microbial activity (reduce bio-kinetic rate) in anaerobic digestion. However such reductions may arise from a number of sources, including loss of enzyme activity and loss of thermodynamic driving force. In this work the theoretical thermodynamic approach was used to describe the inhibitory effects of sodium ion in acetoclastic and hydrogen utilizing methanogenesis. RESULTS: This work takes a modeling approach to investigate thermodynamic limitation by free energy, enthalpy and entropy analysis. Simple free energy analysis provides no evidence for thermodynamic limitation arising from even very high concentrations of sodium ion. However, entropic analysis suggests that increasing concentrations of sodium ion result in the loss of spontaneity. The loss of spontaneity arising from the presence of sodium ions is not directly related to the rise in equilibrium pH, which may also occur in the presence of sodium ions. The results also indicate that hydrogenophilic methanogens are less likely to be affected by the presence of sodium ion than their acetoclastic equivalents. CONCLUSION: From the thermodynamic perspective, the model advises that when the supply of thermal energy is sufficient, sodium inhibition under anaerobic conditions can be virtually completely overcome. The thermodynamic model also provides a design tool with which the stability of methanogenesis in response to feedstock mixtures containing sodium ion and a carbon source such as HAc can be investigated. (c) 2012 Society of Chemical Industry