Electron transfer is studied in a solvent having two independent sets of polar modes. One set modulates the transition matrix element, Delta(0), the other changes the energy at the donor and acceptor sites. The formal solution for the transition probability, P(t), is found in the framework of a modified spin-boson approach when arbitrary driving forces modulate both the transition matrix element and the bias. The general formal solution is analyzed within the noninteracting blip approximation for both types of modes. The transition probability difference is governed by an integro-differential equation with the time-dependent transition matrix element. In the absence of the driving forces, the rate constant increases, decreases, or is independent of temperature at different values of the reaction heat. For small Delta(0), the reaction rate is due to the interaction with modulation vibrations. Depending on the values of the parameters, Gamma(epsilon) exhibits: (a) a four-maxima dependence: or (b) complete insensitivity of epsilon in a broad range of biases. When strong electron-modulation mode interaction occurs, the equilibrium constant reveals the Gibbs＇ behavior with an effective reaction heat, epsilon(eff) = epsilon + (h) over bar omega(0), where omega(0) is the frequency of the local modulation mode. The theory is applied to two different types of the modulation modes: (a) a polar modes Debye spectrum; (b) a quantum (or classical) local vibration with strong dissipation. (C) 2000 American Institute of Physics. [S0021-9606(00)51113-1].