Mesoporous titanium dioxide films were electrophoretically deposited from colloidal suspensions in water on interdigitated microelectrode arrays and also on conducting glass substrates. Even unsintered (just dried) TiO2 films exhibit already usual photocurrent-voltage behavior. Due to the reduced ohmic losses typical of the microelectrode arrays, the effect of the ionic strength of the aqueous electrolyte upon photooxidation of an uncharged hole scavenger (methanol) could be examined. A consistent slight increase of the saturation photocurrent accompanying decreasing concentration of the supporting electrolyte was observed. Measurements performed in various water/methanol mixtures showed a marked continuous increase of the photoresponse with increasing concentration of the dominant hole scavenger (i.e., methanol), suggesting its adsorption as the rate-determining step of the photooxidation process. Close proximity of the TiO2 microanodes and Pt microcathodes, separated by 5 mum gaps, enabled us also to monitor the interplay between the oxygen reduction and the photooxidation of methanol. Accordingly, the presence of oxygen in the solution caused a dramatic decrease of the photocurrent apparently reflecting the effect of hydrogen peroxide, formed at the microcathodes, acting as particularly efficient scavenger of the conduction band electrons at the TiO2 microanodes. In fact, the observed methanol oxidation photocurrents were only slightly affected by the presence of oxygen when the microcathodes were disconnected and replaced by a few millimeter distant external macrocathode. Experiments involving solutions of a large series of organic compounds in pure water revealed a variety of behaviors going from a highly efficient photooxidation to a virtual suppression of the photocurrent. Apparently, the compounds belonging to the latter category, such as acetone and acetophenone, undergo strong adsorption at the TiO2 surface blocking, when present in large concentration, most of the active sites on the surface of the photocatalyst.