In this paper we present a theoretical study of the kinetic isotope effect (KIE) on outer-sphere electron-transfer (ET) reactions, which provides one of the manifestations of quantum effects in such systems. The KIE arising from the deuteration of the ligands in the first coordination layer originates from frequency changes and from distortion of the equilibrium configurations of the totally symmetric stretching modes for the motion of the ligands in the two oxidation states. On the basis of model calculations, we assert that the dominating contribution to the KIE originates from the changes in the ion-ligand equilibrium configurations accompanying the ET process. The KIE, which is expressed in terms of the ratio of the rate constants, kH/kD, is found to be normal and small at room temperature, being kH/kD = 1.26 for the Co(NH3)62+-Co(NH3)6 3+ exchange and kH/kD = 1.12 for the Co(NH3)63+-Cr(bpy)32+ exchange, both calculated at 25°C. The calculated KIE kH/kD = 1.12 for the Co(NH3)63+-Cr(bpy)32+ exchange at 25°C is significantly lower than the experimental value of kH/kD = 1.36. We have examined the dependence of the KIE on the energy gap, demonstrating that kH/kD exhibits a maximum for symmetric ET reactions. A dramatic temperature dependence of kH/kD is predicted with kH/kD exhibiting a very large temperature-independent value at low temperatures, decreasing with increasing temperature, while the isotope effect is completely eroded in the high-temperature classical limit.