Abstract Previous studies have shown that popping volume of popcorn is affected by water content, and that popping volume increases with an increase in water content up to an optimal value, and then the volume decreases with any additional moisture. The aim of this study was to explore the mechanism responsible for the peak in popping volume as a function of water content. Popcorn was equilibrated to various water contents from 6.7 to 16.5 g/100 g d.b. over salt solutions ( a w0.33–0.82), and popped by an air popper. Maximal popping volume occurred at a water content of 15.5 g/100 g d.b. ( a w=0.745 at room temperature) while the popping temperature dropped with increasing water content. Thermal analysis of the popcorn by DSC showed no significant effect of the moisture on the melting temperature or T g of the unpopped endosperm. However, the decrease of the pericarp melting temperature correlated with an increased water content. The results suggest that elevated water contents cause a rubbery collapse of the pericarp at lower temperature. The temperature of this melting event decreases by ∼7 °C over the 6–16.5 g/100 g moisture range. Thus, as the water content increases, the pressure in the kernel at the popping moment is lower, causing less expansion and lower final popped volume.