The free energy dependence on the fluorescence quenching of 2,4,6-triphenylthiopyrylium tetrafluoroborate (TPTP) by a variety of halogenated benzenes, toluenes, and anisoles were studied in acetonitrile. The k(q), values calculated using Delta G double dagger from the Levine expression and the k(d) value of 5.4 x 10(10) M-1 s(-1). from the Smoluchowski expression are in good agreement with the experimental k(q) values. The observation of the TPTP. signal at 550 nm and the correlation of k(q) with Delta G(et) expound the possibility of an electron-transfer mechanism. The radical yield and intersystem-crossing rate constants are evaluated using flash photolysis techniques. The influence of sulfur atom in the triplet induction is reflected in the intersystem-crossing rate constant and radical yield values. The recombination rate constants k(b) obtained from the radical yield values are compared with the k(b) values calculated using the semiclassical expression.