The objective of this research was to study the effect of water activity and glass transition temperature on the fat hydrolysis in a food model system. The model system was prepared with tapioca starch, casein, palm oil and sugar as 58, 14, 16 and 12 g/100 g model matrix, respectively. Hydrolysis reaction was accelerated bycommercial lipase at six levels of water content and water activity. Moisture sorption isotherm was obtained using isopiestic method while monolayer value was determined by BET equations. Glass transition temperature was determined from amorphous ingredients of starch and casein. Hydrolysis reaction showed a significantincrease above the monolayer value at 3.55 g water/100 g solid and aw 0.19. Hydrolysis occurred even at the glassy state of the model system. The role of water in the hydrolysis reaction is more related to the water activity concept rather than glass transition concept.