Water vapor pressure for cornstarch and popcorn grits at a temperature of between 100 and 180-degrees-C was determined by measuring vapor pressures generated at various moisture contents in a heated, closed system. At a higher temperature, fugacity of water increased with increasing temperature probably because of melting or partial melting of starch. The apparent heat of sorption for cornstarch changed significantly with moisture content and temperature. Noncondensible gases were produced by heating popcorn grits to temperatures above 150-degrees-C. It is not accurate to estimate vapor pressure based on the heat of sorption or extrapolation of currently used empirical isotherm equations to a temperature above 100-degrees-C. Vapor pressure inside popcorn at the instant of popping are roughly 758 to 827 kPa (110 to 120 psia).