The photoisomerization of trans-caffeic acid to cis-caffeic acid has been studied in the presence of N-2 in homogeneous aqueous solutions and in suspensions of various TiO2 catalysts. The results supported the hypothesis of an energy transfer process from TiO2 to the substrate due to the recombination of the photogenerated electron-hole pairs. The differences among the measured photostationary [cis]/[trans] ratios have been attributed to the different physico-chemical properties of the catalysts. In particular, the lowest ratio measured in the presence of Merck TiO2 was ascribed to the very low adsorption of trans-caffeic acid onto the surface of this sample. In the presence of O-2 and at alkaline pHs, cis-caffeic acid cyclized to esculetin both in the absence and in the presence of irradiation. (C) 2015 Elsevier B.V. All rights reserved.