Two protonation equilibria (pK_a at 3.7 and 7.4) have been identified for the dye, bis(2,4-dihydroxyphenyl)-squaraine in the ground state. These various protonation forms exhibit widely different absorption properties. The singlet excited-state behaviour of this dye has been characterized by picosecond laser flash photolysis. The fluorescence quantum yield and the excited singlet lifetime of the unprotonated dye (ϕ_f= 0.092, τ_s= 740 ps) are considerably higher than that of the neutral (ϕ_f= 0.037, τ_s= 130 ps) or the protonated (ϕ_f= 0.01, τ_s⩽ 100 ps) forms of the dye. The triplet excited state has been characterized by both direct excitation and T–T energy transfer methods. The triplet excited state has an absorption maximum around 430 nm and a lifetime of ca. 100 µs in ethanol. Photosensitization of a thin microporous TiO₂ semiconductor film has been carried out by adsorbing the squaraine dye from an ethanolic solution and exciting it with visible light. An incident photon-to-current conversion efficiency of 0.05% at 500 nm has been obtained for the dye-modified TiO₂ electrode.