The influence of hydroxyl radicals on the photocatalytic performance of synthesized TiO2 resulting from the addition of water was studied. Two TiO2 samples were prepared using the sol-gel method followed by calcination at 600°C. The aim of this work was to study the effect of water on the photocatalytic activity of a model compound – formic acid (FA) – by comparing between the titanium oxide synthesized with water (TiO2-600-w) and without water (TiO2-600). The TiO2-600-w, consisting of anatase and a little rutile, was the most active catalyst, whereas pure anatase (in case of TiO2-600), showed low photocatalytic efficiency. In this study, we confirmed that the filling of the TiO2 surface by hydroxyl radicals could produce better photocatalytic activity by reducing e−/h+ recombination. It is notable, however, that with TiO2-600-w, the degradation rate (found under UV) was about 4–5 times greater than that obtained with TiO2-600. This result suggests that the presence of water was very important in the synthesis of TiO2 and brought a dramatic gain in the photocatalytic response as well.