The development of relationships between the pH of a heating medium and the thermal resistance of contaminant microorganisms is important and of a public health significance. A number of mathematical models have been presented in recent years, including that of Mafart and Leguérinel (1998) . However, in this This model , the effect of possible interactions between temperature and pH on D-values was not assessed. The consequences of ignoring interaction terms in models has 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 of zpH , 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).