Photoinduced charge-transfer reactions between adsorbed pyrene and N,N’-dimethylaniline (DMA) in porous silica gel are examined by fluorescence quenching and transient absorption techniques. The quenching on silica surfaces is found to be diffusion controlled and gives rise to exciplex formation on silica surfaces. Such a Langmuir-Hinshelwood type of surface reaction is described quantitatively using a two-dimensional kinetics model. Surface diffusion of DMA is found to be thermally activated and related to the hydrogen-bonding interaction between DMA and the surface hydroxyl groups. Properties of exciplex emission such as the position of the maximum and the quantum yield are used to characterize the surface environment. Deactivation of the exciplex on silica surfaces via charge recombination is understood in terms of photoassisted The low yield of ionic products from dissociation of the contact ion pairs of the exciplex (1)(Py(.-)DMA(.+))* is attributed to the lack of solvation and the low mobilities of ionic species on the surface. It is found that coadsorption of alcohols facilitates the charge separation.