The photochemistry of urocanic acid is investigated theoretically by means of time-dependent density functional theory. The topology of the potential energy surface along the isomerization pathway and close to the Franck-Condon region is investigated and consequences for the photoisomerization reactions are outlined. A recently published supersonic jet spectroscopy study is reinterpreted in the light of these and earlier theoretical results to give a clear picture of the gas phase photochemistry. It is found that the photochemistry of the two isomers is fundamentally different, in contrast to the situation in solution. (C) 2003 Elsevier Science B.V. All rights reserved.