Time-resolved absorption spectroscopy with similar to40 fs time resolution was applied to the S-1 state of cis-stilbene in solution. An oscillatory signal caused by similar to220 cm(-1) wavepacket motion was clearly observed. Its damping rate was much faster than the isomerization rate and insensitive to the change of the solvent. The obtained ultrafast pump-probe data showed that the dephasing of the initial coherent wavepacket motion first takes place rapidly, followed by the similar to1 ps isomerization reaction. The significance of the similar to220 cm(-1) mode, intramolecular vibrational energy redistribution (IVR) process, and high anharmonicity of the S-1 potential surface in the initial stage of cis-stilbene photochemistry is discussed. (C) 2004 Elsevier B.V. All rights reserved.