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The authors present a new method for the qualification of photocatalytically “active” surfaces (e.g. TiO 2 coated glasses) comprising a solid-state luminescent dye, which is thoroughly characterized and investigated regarding its behavior on different manufactured photocatalytically active substrates versus inactive reference materials. The dye is an europium(III) complex showing the typical intense 5 D 0 → 7 F 2 transition with an emission wavelength maximum of 615 nm upon excitation at 350 nm. The dye is deposited as a thin-film reaching from 10 nm to 100 nm onto the substrates using an ultra-high vacuum (UHV) deposition technique. This system is excited with 20 W/m 2 at 365 nm with a LED and the luminescence is time-dependently monitored with a spectrofluorimeter. The dye's luminescence shows only a slight decrease on inactive substrates such as glass or silicon wafers, while showing a significant decay on photocatalytically active TiO 2 substrates. A possible mechanism for the luminescence decay is proposed. This direct method of luminescence degradation is highly sensitive and reproducible. It represents a promising option to be considered in standardization efforts in the field of photocatalysis.