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 zpH value (zpH is the distance of pH from a reference pH*, which leads to a ten fold reduction of D-value). The type of organic acid used for acidifying the heating medium, influences the zpH value. For nine organic acids, a linear relationship between the calculated zpH value and its lower acid pKa 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 zpH value.