The development of relationships between the pH of a heating medium and the thermal resistance of contaminant micro-organisms is important and of public health significance. A number of mathematical models have been presented in recent years, including that of Mafart and Leguérinel (1998). However, in this model the effect of possible interactions between temperature and pH onD-values was not assessed. The consequences of ignoring interaction terms in models have been assessed, and a comparison with Mafart's model that includes an interaction term showed that interaction terms can be neglected and that Mafart's model can be used in thermal process calculations. It appears possible to adopt a standard value ofZpH, for example the 3.6 value, and the conventional concept of biological destruction value L(T) (ratio of the sterilization value and the exposure time at a fixed heating temperature) may then be extended to L(T,pH) (the same ratio at a fixed temperature with a fixed pH of the heating menstruum).