The kinetics of electron transfer on silica gel between the anthracene radical cation and the electron donors triphenylamine (TPA) and N,N,N＇,N＇-tetramethyl-1,4-phenylenediamine (TMPD) have been investigated using the technique of diffuse reflectance laser flash photolysis. The mean rate of decay of the anthracene radical cation, determined as the maximum of the rate constant distribution, correlates with the surface concentration of the electron donor in both cases up to a loading of 2 mu mol g(-1). In the case of triphenylamine as donor, using 355 nm excitation, plots of mean electron-transfer rate constant versus electron-donor concentration deviate positively from Linearity at higher donor concentrations. In the case of TMPD, linearity is preserved at higher loadings with 355 nm excitation, where TMPD does not absorb appreciably and is not therefore directly photoionized, but again a positive deviation ensues with 266 mm excitation that causes direct photoionization of TMPD. In both cases the increased rate of decay of the anthracene radical cation correlates with appreciable direct photoionization of the electron donor. The enhanced rate of decay is explained as being due to donor photoionization increasing the local concentration of electrons on the surface and electron diffusion competing efficiently with donor diffusion leading to increased anthracene radical cation decay. In addition, we have demonstrated the formation between anthracene and TPA coadsorbed on silica gel of an emissive exciplex and have found no evidence that relaxation of this exciplex results in radical ion pair formation.