When benzene was oxidized on TiO2-photocatalysts, muconaldehyde as well as phenol was identified as a product. Muconaldehyde was not produced from phenol, indicating that muconaldehyde and phenol are produced as initial stable intermediates by independent pathways. The amount of muconaldehyde produced in the reaction solution was low because it was rapidly oxidized on the photocatalyst. The contributions of the two pathways to the mineralization of benzene were analyzed by isotopic tracing methods using labeled benzene ((C6H6)-C-13) and labeled phenol ((C6H5OH)-C-13). The results showed that a larger part (60-70%) of benzene is initially converted to muconaldehyde. The percentage was dependent on the properties of TiO2 powders. It was also found that anatase-form powders show higher activity for oxidation of benzene than rutile powders do, whereas rutile-form powders show higher activity for oxidation of phenol than anatase powders do. (C) 2011 Elsevier B.V. All rights reserved.