Diffusional and electrostatic effects on the apparent maximum reaction rate V-m(app) and the apparent Michaelis constant K-m(app) were investigated theoretically for a system in which an enzyme immobilized on the external surface of a solid support catalyzes a reaction according to Michaelis-Menten kinetics. In such a system, the dependence of V-m(app) and K-m(app) on the substrate concentration can be expressed analytically. When the support and substrate carry charges of the same sign, resulting in a repulsive force between them, both V-m(app) and K-m(app) decrease with increasing substrate concentration, but they never decrease below the respective intrinsic values. On the other hand, when the support and substrate carry charges of opposite sign and therefore an attractive force occurs, V-m(app) decreases towards its intrinsic value, while K-m(app) decreases to values below its intrinsic value in the region of high substrate concentration.