Photoinduced electron transfer (ET) reactions of quinones (A) and tertiary aromatic amines (D) both adsorbed onto porous silica (14 nm pore size) were studied by diffuse-reflectance laser flash photolysis technique. Both diffusion-controlled dynamic and Perrin type static quenching of (3)A by D were observed. Static quenching results in formation of triplet radical ion pairs (RIPs). RIPs decay via an intersystem back electron transfer (ET). The ET kinetics are discussed in terms of two formalisms : a first-order law with Gaussian distribution on the free energy or by a fractal-like analysis with time dependent first-order rate constant k(t) = k’(f)t(-h). The heterogeneity constant, h, increases with the increase in average rate constant of reaction in accordance with the theoretical predictions for lower dimensional and fractal media. The back ET is a reaction-controlled process at early times and a diffusion-controlled one at times longer than 0.5 mu s. The dependence of the average rate constant of back ET and of k(t) at early times on the ET free energy is bell-shaped. This can be quantitatively described in terms of the single quantum mode model of the nonadiabatic ET theory with a higher value of the reorganization energy of environment (0.9 eV) as compared to that in moderately polar solvents (other parameters being the same). The bell-shaped energy gap dependence demonstrates that adsorbed RIPs appear to experience a strongly polar environment.