Characteristics of a light-addressable potentiometric sensor (LAPS) based on silicon with Ta2O5 dielectric are reported. The pH sensitivity obtained is 45 mV/pH unit in the pH range from 4.01 to 7.5 (for measurements conducted in citrate buffer solutions) and 55 mV/pH unit over the pH range from 7.5 to 8.9 (for measurements conducted in Tris-HCl buffer solutions). The kinetic characteristics (K-m, V-m, pH-profile) of different enzymes, i.e., glucose oxidase, alpha-chymotrypsin, butyrylcholinesterase, and urease, have been measured under homogeneous conditions. The values of the Michaelis constant obtained are very close to analogous data described in the literature. Biosensors were prepared by immobilization of the enzymes on a pH-sensitive matrix. Two methods of immobilization were used: incorporation into a hydrophilic matrix of bovine serum albumin (BSA) and incorporation into a hydrophobic matrix of modified polyethylenimine (PEI). It is demonstrated that LAPS can be used for recording the enzymatic reactions in organic media (mixture of the solvents).