A rate constant expression for charge transfer reactions mediated by flexible bridges is presented as a series of terms of decreasing importance. The leading term corresponds to the static limit obtained from the Condon approximation. Corrections due to finite time fluctuations are evaluated explicitly, assuming a Gaussian shape of the coupling autocorrelation function and the Marcus model with a one-dimensional harmonic thermal bath. The use of this model for the interpretation of experimental data and the expected magnitudes of the fluctuation effects are discussed. (C) 2003 American Institute of Physics.