AbstractThe efficiency of aromatic ketones as singlet‐oxygen (1O2(1Δg)) sensitizers can vary considerably with the electronic configuration of their lowest triplet state and the solvent used. Near‐infrared measurements of tie luminescence of singlet oxygen have shown that the quantum yield of singlet‐oxygen production (ΦΔ) by 1H‐phenalen‐1‐one (1) is close to unity in both polar (ΦΔ= 0.97±0.03 in methanol) and non‐polar solvents (ΦΔ= 0.93±0.04 in benzene). Analysis of the absorption spectra of the ground state and phosphorescence measurements show that the lowest singlet and triplet states have dominant π, π* electronic configurations. The quantum yield of intersystem crossing (ΦISC) of1, determined by laser flash photolysis (partial‐saturation method), is equal to unity. In comparison with other aromatic ketones, these parameters are important for the discussion of the surprisingly high ΦISCof1and the efficient energy transfer from its triplet state to molecular oxygen. The 1H‐phenalen‐1‐one (1), being one of the most efficient singlet‐oxygen sensitizers in both polar and non‐polar media, could be used as a reference sensitizer, in particular in the area of