This experimental work studies photocatalytic degradation of phenol. The semiconductors used were zinc oxide (ZnO) and three commercial titanium oxides (TiO2) (Degussa P25, TiO2-A1 and TiO2-A2). The rate of removal was greater for Degussa P25 and ZnO. To understand the difference between photocatalytic activity of the three titanium oxide powders, a measure of the grain diameter under MEB as well as analyses by X-ray were made. The anatase form was identified with a difference in the diameter of grains and crystallites. The diameter of the grain was practically equal to that of the crystallite for the P25, and it was equivalent to three and five crystallites for TiO2-A1 and TiO2-A2, respectively.