The photocatalytic activity of polycryst. TiO2 samples impregnated with functionalized Cu(II)- or metal-free porphyrins has been investigated. A probe reaction, i.e., 4-nitrophenol (4-NP) photodegrdn. in aq. suspension, has been used. The results indicated that the presence of the sensitizers is beneficial for the photoactivity and suggest an important role of Cu(II). A comparison with similar samples impregnated with modified Cu(II)- and metal-free phthalocyanines showed that the presence of porphyrin is more beneficial both for the decompn. rate of 4-nitrophenol and for the disappearance of non-purgeable org. carbon (NPOC). Expts. were carried out by using cutoff and interference filters and a cooperative mechanism involving the photoactivation of both TiO2 and sensitizer is proposed.