Ultrafast absorption changes are recorded for water-soluble cyclic azobenzene peptides containing the photoswitch (4-amino-methyl)-phenyl-azobenzoic acid (AMPB) and a bioactive peptide motif. They can be separated into the fast reactions in the AMPB chromophore and the slower response of the peptide moiety. While the fastest reactions display similar time constants as observed for AMPB peptides dissolved in DMSO the slower reaction dynamics assigned to vibrational cooling and motions of the peptide moiety are faster in water by a factor of up to two. The changes in the reaction times are explained by solvent heat capacity and viscosity. (C) 2004 Elsevier B.V. All rights reserved.