The Williams-Landel-Ferry (WLF) equation is a useful tool in the prediction of temperature-induced physical changes in foods that are exposed to specific food processing and storage conditions, because so many deteriorative processes are diffusion controlled. In food polymers, viscoelastic properties are a strong function of the key plasticizers present in foods, such as water. However, WLF constants as a function of water content and water activity are not available in the food literature. In this research, we investigated the WLF properties of soy flour as a function of water activity and elucidated the effect of water on the magnitude of WLF constants. Based on a time-temperature superposition principle, shift factors (a(T)) were obtained, which were used to estimate WLF constants. WLF constants for soy flour were shown to be material specific and different from the "universal values."