The kinetics of an intramolecular proton transfer cycle, in particular the population of the various transient states, is investigated by a new experimental technique : degenerate four-wave mixing in combination with excitation of the molecules by picosecond (2 x 10(-11) s) UV pulses. The molecular hyperpolarizability gamma((3)) of different excited electronic states is measured at five spectral positions between 10 000 and 30 000 cm(-1). Resonance enhancement up to 3 orders of magnitude near electronic transitions is observed. The high sensitivity of gamma((3)) to molecular resonances allows the study of extremely short-lived states. As an example, data are presented on the dynamics of the intramolecular proton transfer reaction in 2-(2’-hydroxyphenyl)benzothiazole where three transient states are elucidated.