Abstract — The photophysics and photochemistry of organic molecules (aro— matics , aza—aromatics , diphenylpolyenes , azobenzene , triphenylmethane dyes, thioindigo) adsorbed on metal oxides (alumina, silica, thoria, ti—tania) were investigated by steady—state and time—resolved diffuse ref lec—tance and luminescence spectroscopy.The fluorescence decay curves of almost all adsorbates are non—exponen—tial. This effect is discussed in terms of different polarizing surfacesites, aggregation, and interference between directly emitted and scat—tered fluorescence radiation.A method is elaborated to determine rate constants and quantum yields ofphotoreactions in strongly scattering rigid media by photometric measure—ments using the model of radiative transfer. The method is applied toquantify internal and translational mobilities of adsorbed photoexcitedmolecules, ring closure reactions, bimolecular photoprocesses, and chargetransfer between the adsorbent and the adsorbate. INTRODUCTION Photochemical reactions are conventionally studied in homogeneous phases and, as far as pos—sible, in ideally stirred media. These conditions, however, are not to be realized in manysystems of biological or technical interest. The medium can be strongly light—scattering andhighly viscous, and the reacting species can be very inhomogeneously distributed in it (e.g.in membranes or in the adsorbed state). Compared with the total activities in photochemicalresearch, the heterogeneous phenomena have received up to now only moderate attention. Butthe attention is strongly growing —