The third armonic (355 nm) of a pulsed Nd-YAG laser has been used to excite pyrene in micellar solutions of Triton X-100. Fluorescence quenching by nitromethane and triethylamine of the excited state of pyrene in Triton X-100 has been studied. Quenching rate constant values of 4.94 ± 0.13 × 108M−1s−1and 1.50 ± 0.04 × 108M−1s−1for deactivation by nitromethane and triethylamine, respectively, have been measured. The values obtained are discussed in terms of the interface permeability.