Kinetic analyses of geminate radical escape yields in terms of a simple ("" exponential ÏÏ) reaction scheme with Ðrst-order rate constants of separation and geminate recombination have been widely used in the literature, e.g. to evaluate rate constants of reverse electron transferHere we demonstrate the limited value of such (k~e t ). rate constants by formally analysing, in terms of the exponential model, the di †usion coefficient (viz. viscosity) dependence of the radical escape yield as theoretically calculated in the framework of di †usion-dependent electron transfer theory (uniÐed treatment of non-contact photoinduced forward and geminate reverse electron transfer). It is shown that, while the true electron transfer rate constant is kept constant, the apparent rate constant from the exponential model undergoes a wide variation as a function of di †usion coefficient and k~e t the rate of spin conversion. Nevertheless, the function represented in a double log plot for various rates k~e t (D) of spin conversion provides a useful map suitable to assign characteristic regions of di †usional, spin and reaction control of the geminate process. As an application to real systems the experimental example of the system is reconsidered. Here a magnetic Ðeld e †ect on the dependence is [Ru(bpy) 3 ]2`/methylviologen k~e t (D) useful to corroborate the non-contact formation of the radical pair in the photochemical forward electron transfer reaction.