The water activity (alpha w) of the liquid phase is investigated by means of 1H NMR for both monophasic and biphasic systems. The chemical shift or the area of the signal of the hydroxylic hydrogens is compared to calibration curves obtained from mixtures equilibrated at different water activities, thus allowing determination of the alpha w of the system. The chemical shift varies linearly as a function of the alpha w of the system. Through simple calculation the hydroxylic hydrogen concentration obtained from the area gives the water concentration in the medium; the sorption curve is thus obtained through NMR. The application of this method to a biphasic system composed of a liquid phase (an equimolar mixture of acid-alcohol) and a solid phase (a lipase), equilibrated separately at two different water activities, gives information on the water exchange between both phases during the lag phase of the esterification reaction. Light and slow water exchanges are observed from the solid phase at high alpha w toward the liquid phase at low alpha w. The lag phase of the esterification reaction is too short for the water activity equilibrium to be reached before the reaction starts.