Enzyme-potentiometric sensors in which the chemical conversion of the analyte leads to the formation of an acid and/ or a base can often display complex response characteristics. In these sensors, the electrochemically monitored species is usually the H + ion (pH-based sensors). However, in some cases the conjugate-ion of the H' (or OH-) ion of the acid (or base) produced can also be monitored-a specific example being the urease-NH4+ sensor. The response of both types of sensors is strongly affected by: (1) the degree of dissociation of the products and their transport properties in the enzymic film, (2) the amounts of pH-buffers present in the test solution, (3) the test solution's pH, and (4) the diffusion coefficients of the various species. In this article, a previously developed theoretical model for pH-based sensors-in which the differences in diffusivities of t h e various species were ignored-is generalized to accommodate for such differences, and extended to the latter of the above two types of sensors. It is shown that when the sensor operates under analyte diffusioncontrolled conditions, the response of either type of sensor can be predicted by a simple algebraic equation which is independent of the actual kinetics of the enzymic reaction.