The effect of 2-phenylindole (2PI) concentration in p-xylene upon the fluorescence decay time was investigated by frequency-domain fluorometry. The fluorescence decay of a dilute (10~ 5 M) 2PI solution in p-xylene at 20 °C is single exponential with the mean decay time t = 1.87 ns and practically independent of the observation wavelength (A obs) in the fluorescence band. The fluores-cence decay in a highly concentrated solution (6 x 10" 3 M) is heterogenic and depends on A obs . For a given modulation frequency (~ 200 Hz), the fluorescence phase shifts at the longwave observation (440-480 nm) distinctly exceed 90°, thus clearly indicating that the observed (excimer) fluorescence results from a process taking place in the excited state. The fluorescence decays were measured at seven values of A obs (360-480 nm), and all the results were analyzed comprehensively (tj and t 2 are equal for all A obs ; the amplitudes are different). The lifetimes of the excimer state is relatively short (~3.5 ns). Radiative (r e) and non-radiative (t,) rate constants for monomer fluorescence of 2PI in p-xylene were determined using appropriate data from photo-and radioluminescence measure-ments.