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The photocatalytic performances of doped and non-doped TiO₂ photocatalysts (TiO₂-s) were compared under solar and various types of artificial irradiation using phenol as a model contaminant. Non-doped (mainly anatase phase) TiO₂-s had significantly higher photocatalytic efficiency than highly visible-light-active TiO₂-s under natural solar irradiation. To explain these unexpected results, we measured the wavelength dependence of photocatalytic efficiency at six different wavelength ranges (λ = 300–650 nm). For this purpose, UV fluorescence tubes and five LED lights of different colors (violet, blue, green, yellow, and red) were used to activate the photocatalysts. The photon fluxes of the irradiation were measured, and apparent quantum yields were calculated for all irradiation conditions. The highest apparent quantum yield was 1.43% for our own TiO₂ (prepared via flame hydrolysis) under UV irradiation. However, apparent quantum yields were significantly lower (by 1–2 orders of magnitude) in the visible range, even for the most visible-light-active TiO₂.