We investigate electron transfer reactions on diabatic surfaces with different curvatures within the framework of Zusman theory. First, a generalization of the nonadiabatic Marcus-Levich-Dogonadze rate expression is obtained for the case of different forward and backward reorganization energies. Second, we provide a corresponding generalization of the Zusman rate expression which bridges between nonadiabatic and solvent controlled adiabatic electron transfer. The derived analytical rate expressions compare favorably against the precise numerics. A detailed comparison with previous work of Tang is presented. (C) 2002 Elsevier Science B.V. All rights reserved.