A method was developed to determine the water activity (Aw) of food systems in the range of 0.85 to 0.98 Aw. The method is based on the equilibrium moisture absorption of microcrystalline cellulose at a given temperature. Sulfuric acid solutions of known concentration and Aw were used to prepare a standard curve of equilibrium moisture absorption vs. Aw at 35°. A known amount of the standard microcrystalline cellulose was placed in desiccators containing about 50- to 100-g food sample and evacuated for 1.5 min. After 24 hr, the weight gain of the cellulose was measured and the moisture content calculated. Results show that the method is comparable to that of the electric hygrometer and considerably better than the manometric technique.