The features of a biosensor based on a platinum electrode modified with a poly(phenol) film coupled with two enzymes, alkaline phosphatase(AP) and glucose oxidase(GOD), were studied. The modification of the surface of the working electrode decreases interference from ascorbic acid, uric acid and some oxidizable organic materials (e.g. glycine), and decreases the noise of the current. The concentration of substrate, the activity ratio of the two enzymes, the applied potential, and the assembling of biosensor are important for the determination of low levels of phosphate.