the reaction medium affects it? A reliable identification and quantification of benzoic acid, a possible reaction product, by 1 H NMR spectroscopy is complicated due to its low concentration and hydrogen bonding resulting in very broad peaks. In order to obtain more precise data regarding the formation of this compound, we reduced the reaction volume by the factor of 10 and carried out also C NMR studies of the solutions after irradiation (Table ). The irradiation of monophasic benzyl alcohol-benzaldehyde solution resulted in the decrease of initial benzyl alcohol concentration, which can be judged by the peak at 193.3 ppm (Figure ). Benzaldehyde content after light exposure was also reduced, which was most likely due to its further oxidation to benzoic acid (BAc), as confirmed by appearance of a peak at 170.2 ppm (Table , Figure ). On the other hand, the photooxidation in the biphasic benzyl alcohol-benzaldehyde/water mixture does not lead to benzaldehyde overoxidation to benzoic acid and an increase of benzaldehyde concentration with respect to the initial one can be observed (Table , Figure ). The same reaction using an organic solvent (acetonitrile) did not lead to the formation of detectable amounts of H2O2 and resulted only in slight conversion of benzyl alcohol, significantly lower than that observed in solvent-free conditions (Table ).