CO2 and O-2 were the only products of photoelectrooxidation detected on-line during the competitive reaction of formic acid and water in acid solutions on semiconducting TiO2 anatase layers. The mechanistic pathway of oxygen photoevolution at such layers occurs through a rate determining bimolecular reaction as put in evidence by isotope labeling experiments with O-18. This is confirmed by a kinetic model used to fit the isotope labeled transients of mass signals. In accordance with other authors, this rate determining step is discussed as the reaction between hydroxyl radicals to peroxide groups. The CO2 mass signals give evidence that the competitive photoelectrooxidation of HCOOH occurs via hydroxyl radicals formed during photooxidation of water. No incorporation of O-18 in CO2 was detected as HCOOH was photoelectrooxidized on O-18-enriched TiO2 layers. However, this might be due to the high background of the mass signal of the initial compound HCOOH (m/e = 46).