TiO2 samples of different crystal forms and crystal sizes were prepared by calcining the hydrolysis product of titanium tetraisopropoxide at different temperatures. The photocatalytic degradation rate of several compounds over TiO2 was either dependent on the calcination temperature (group I) or independent of it up to 500 degrees C (group II), Group I compounds are sparsely water-soluble, whereas group II compounds are water-soluble. For group I compounds there was a proportional relationship between the degradation rate and crystal size of anatase. A hypothesis explaining the mechanism of the photocatalytic processes of the two groups was proposed on the basis of their different adsorption modes. The addition of H2O2 accelerated the photocatalytic degradation. This effect was more pronounced for rutile than anatase.