The excited state interactions occurring when a three-component system of thioxanthene derived dye TXD/amine/additive (diphenyliodonium salt, CBr4, benzoyl peroxide, cumene hydroperoxide) is subjected to sensitization processes in the visible range, were investigated through time-resolved absorption spectroscopy, spectrofluorometry, and photolysis. The rate constants of the various operative processes were measured together with the values of the fluorescence quantum yields (e.g. phi(f) = 0.75 +/- 0.07 in methanol) and the lifetimes of the singlet excited state of the dye (e.g. 6 ns in methanol). Singlet state quenching by methyldiethanolamine (MDEA) occurs with a rate constant k = 10(9) M(-1) s(-1) in methanol. The reactivity of the triplet excited state of the dye is very low (k = 5.6 X 10(4) M(-1) s(-1) for the TXD/MDEA interaction). The ketyl radicals that arise from the interaction of the singlet state of the dye with the amine, are quenched by such additives as CBr4 (k = 6.7 X 10(5) M(-1) s(-1)), or the onium salts (k = 5.7 X 10(5) M(-1) s(-1)). The calculations of the yields of interaction of the singlet state of the dye with the two components of the system demonstrate that the process of quenching by the amine is the main one (phi(int) = 0.5) compared to that by, e.g., an onium salt (phi(int) = 0.07). Sensitivity of 0.3 mJ cm(-2) obtained in a laser scanning system is also reported.