Nanosecond laser flash photolysis employing transient detection of emission and absorption in combination with pulse radiolysis and quantum theory has been employed to shed light into the kinetics, quantum yields, and mechanisms of the deactivation of the first excited singlet state of 1- and 2-thionaphthols (NpSH(S-1)). In contrast to thiophenols (ArSH(S-1)), the results revealed that the decay of the first excited singlet state of 1- and 2-thionaphthols (NpSH(S-1)) is governed by radiationless internal conversion (Phi(IC) = 0.29-0.46; 0.016-0.190) and intersystem crossing (Phi(ISC) = 0.14-0.15; 0.4-0.6), respectively, with pronounced S-H photodissociation (Phi(D) = 0.40-0.55; 0.35-0.40). Fluorescence as a deactivation channel plays a minor role (Phi(F) = 0.001-0.010; 0.010-0.034). Quantum chemical calculations helped in understanding the formation of naphthylthiyl radicals and rationalizing the differences in the efficiency of intersystem crossing of the 1- and 2-thionaphthol systems.