Characterization of polycrystalline TiO2 bare or porphyrin impregnated powders, used as photocatalysts for the degradation of 4-nitrophenol (4-NP) in aqueous suspension, was performed by time-resolved microwave conductivity (TRMC) measurements and electronic paramagnetic resonance (EPR) and X-ray photoelectron (XPS) spectroscopies. The presence of porphyrin sensitizers, as the metal-free or Cu [5,10,15,20- tetra (4-tert-butylphenyl)] porphyrin, impregnated onto the TiO2 surface improved the photocatalytic activity of the bare TiO2. TRMC measurements indicate that the number and lifetime of the photoinduced excess charge carriers increase in the presence of the macrocycles, and EPR and XPS spectroscopies support the mechanistic hypotheses based on the photoreactivity experiments.