Photoreforming of diols, such as ethylene glycol, proceeds through a sequence of anodic oxidations, which enable the parallel formation of H-2 by reduction of H* at the cathode. The anodic oxidation of ethylene glycol on Rh/TiO2 leads to glycolaldehyde, formaldehyde and acetaldehyde as primary products. Glycolaldehyde is further converted via oxidative C-C-cleavage to formaldehyde and formic acid. Formaldehyde is oxidized to formic acid forming CO2 and H-2. Acetaldehyde is oxidized to acetic acid, which decarboxylates to CO2 and CH4. Two catalytically active sites are proposed. On terminal Ti-IV-OH groups, oxygenates are oxidized via direct hole transfer to alkoxy-radicals prior to beta-C-C-bond cleavage. Bridged [Ti center dot center dot O-center dot center dot center dot Ti](+) sites, in contrast, cleave a C-H bond, forming carbon centered radicals, which are further oxidized by transferring an electron to the conduction band of the semiconductor. On Rh/GaN:ZnO, glycolaldehyde is the main product, forming higher oxidized C2-oxygenates in turn by reaction with free oxygen radicals formed as product of OH- photocatalytic oxidation. The overall rates of photoreforming and, hence, H-2 evolution, depend mainly on the surface concentration of the compounds which are oxidized, while the nature of the oxygenate is of less importance. (C) 2016 Elsevier Inc. All rights reserved.