A mathematical model of steady-state and non-steady-state responses of a pH-based potentiometric biosensor immobilizing organophosphorus hydrolase was developed. The model is based on non-stationary diffusion equations containing a nonlinear term related to the Michaelis-Menten kinetics of an enzymatic reaction. An analytical expression for the substrate concentration was obtained for all values of parameter a (Thiele modulus) using the homotopy perturbation method. From this result, the concentrations of the deprotonation products of an organophosphodiester (PhH, ZH and AH) were obtained. Our analytical results were compared with available simulation results. A satisfactory agreement with the simulation data is noted.