Photoanodic (water and methanol oxidation) and cathodic (oxygen reduction) processes occurring at illuminated (with near - UV light) bulk and particulate anatase and rutile TiO2 films were analysed. (Photo) current-potential curves recorded for both anodic and cathodic processes point to large differences in activity between the two forms of TiO2. In particular, in the presence of various organic compounds, the photocurrent onset potential of anatase tends to shift negatively (sometimes as much as 0.3 V), thus increasing drastically the difference with respect to the potential of oxygen reduction. The situation is quite different for the rutile samples, the photocurrent onset potential of which appears practically unaffected by the addition of organic species to the solution and remains close to the onset potential of oxygen reduction. The reverse reaction with respect to the photoanodic process, involving species generated by the photo-oxidation of water and OH- ions, is shown to play often an essential role in determining the photocurrent onset potential (i.e. the operating mixed potential) of the photocatalyst. This is the prevailing situation for rutile but is much less frequent for anatase. In fact, a number of species, especially among organic compounds, appear able to compete efficiently for positive holes with OH groups adsorbed at the anatase surface, leading to a virtual suppression of the reverse reaction. In such a case, the reduction of oxygen present in the solution becomes the main cathodic process. This description is consistent with the fact that, in most cases of real photodegradation processes, the anatase form of TiO2 exhibits much greater activity than the rutile form.