The preparation of the capacitive biosensor for monitoring the activity of enzymes (lipases, phospholipases, etc.) hydrolysing water-insoluble substrates into water-soluble products is described. The sensor is based on a sandwich-like structure : Au/S(CH2)(17)CH3/substrate/electrolyte. Hydrolysis of the substrate leads to the formation of water-soluble products and desorption of these compounds from the electrode. When the product formation is the rate-limiting step of this process, the desorption rate is determined by the enzyme activity. The desorption can be easily monitored as an increase of the electrode capacitance. By following this principle it was possible to develop a sensor monitoring phospholipase A, activity even at the 50 pg ml(-1) level.