Aims: To determine the effect of reduced water activity (a(w)) on thermal inactivation of Salmonella serotype Typhimurium at different temperatures and its mechanism. Methods and Results: D-value determinations at a range of different temperatures showed that heating at reduced aw (0.94, produced by addition of glucose or sodium chloride to nutrient broth) was protective at temperatures above 53-55 degrees C but sensitizing below this temperature. Using selective enumeration media to determine injury, it was shown that at lower heating temperatures cells survived at high aw with cytoplasmic injury whereas at low aw these cells were killed. At higher temperatures ribosome degradation was a more important cause of death and was inhibited by low aw heating media thereby providing greater heat resistance. Conclusions: The observed change in behaviour reflects the different reactions responsible for thermal death at different temperatures and their different response to reduced aw. Significance and Impact of the Study: This work qualifies the previous assumption that reduced aw is protective and suggests that the efficacy of low temperature pasteurization regimes may be increased by reduced aw.