A model is proposed to describe the influence pH on the heat resistance of Bacillus cereus spores. In addition to the conventional z value, the effect of pH on the thermal resistance of spores is characterised by a z p H value (z p H is the distance of pH from a reference pH*, which leads to a 10-fold reduction of D value). The type of organic acid used for acidifying the heating medium, influences the z p H value. For nine organic acids, a linear relationship between the calculated z p H value and its lower acid pK a is observed. This relationship showed that the acid form (dissociated or undissociated) modifies the thermal spore resistance in addition to the H + ion. The influence of acetic acid concentration on the D value at pH 7 shows the protective effect of the dissociated acid form on the heat resistance of spores. The acid concentration in the medium modified the heat resistance of spore and the z p H value.