Water activity (a(w)) plays an essential role in food science and technology in optimizing processes (i.e. drying) and evaluating the microbiological, chemical and physical stability of foods. Accurate and precise measurement of a(w)is also required in view of the new trend to minimal processing of foods. A thermostated vapour pressure capacitance manometer (VPM) apparatus, based on the theory of Benado and Rizvi, was designed and built. It consisted of two flasks; one for sample and another for desiccant (CaSO4), connected by means of two solenoid valves to a tubing manifold, connected to a pressure transducer; and to a vacuum pump by means of a solenoid valve. Reference samples (i.e. pure water; saturated salt solutions and microcrystalline cellulose at different moisture contents) were used to validate the VPM at 25 degrees C. Results showed an accuracy of +/-0.01 a(w) and a standard deviation of +/-0.01 a(w) Based on this study, the evacuation time and the time of sample equilibrium after evacuation were identified as the two critical points of the VPM for for accurate measurements.