Cis-trans photoisomerization of a photoactive yellow protein chromophore model, the deprotonated trans S-phenyl thio-p-hydroxycinnamate, is studied in aqueous solution by subpicosecond transient absorption and gain spectroscopy. The excited-state deactivation is found to involve the formation, in 1.7 ps, of an intermediate state which decays in 2.8 ps. A persistent bleaching signal is observed at longer times indicating that the excited state not only relaxes to the ground state but also partly forms a stable photoproduct, possibly the cis isomer. This behavior is analogous to that of the native photoactive yellow protein. (C) 2002 Elsevier Science B.V. All rights reserved.