We report a sub-picosecond time-resolved fluorescence spectroscopic study of different cinnamic acid crystals, model systems for solid-state photodimerization reactions. For alpha-cyano-4-hydroxycinnamic acid (alpha-CHC), we identify the emission spectra of both monomers and dimers, allowing us to directly probe the photoinduced dynamics of both species. The dimerization occurs on a timescale of 10 ps and results in a long-lived dimer product, stable for hours. For sinapinic acid, we find an extremely fast, sub-picosecond dimerization reaction and a short-lived dimer. This first sub-picosecond time-resolved dimerization study in cinnamic acid crystals provides a new basis for relating their structural properties and microscopic reaction dynamics. (c) 2007 Elsevier B.V. All rights reserved.