A new type of surface acoustic wave impedance sensor based on a conductance-surface acoustic wave frequency reponse was applied to assay the activity of acid phosphatase. Using cytidine-5′-monophosphate (5′-CMP) as a substrate, the Michaelis constant and the corresponding maximum initial rate were estimated to be 4.13×10−3and 9827 Hz/min, respectively. A linear relationship between frequency response and enzyme concentration was obtained in the range up to 250 μg/ml of acid phosphatase, at 25°C and pH 4.5. The experimental detection limit of acid phosphatase (within 1 min) was 0.9 min/ml and the recovery of the sensor system ranged from 94% to 106% (n = 6). Some other factors such as temperature, pH value and the effects of some metal ions were also discussed.