with furfuryl alcohol as probe compound at an initial concentration of 250 mM revealed only minor O isotope fractionation, that is a change in δ O values from 25.3 to 27.5 despite a similar degree of O 2 consumption (entry 3, Table ). Note that the maximum change of δ O observed in a control experiment was 0.7 which we use as lower bound for negligible isotope fractionation comparison. To that end, all of the photochemical experiments shown here reveal some extent of O isotope fractionation which we can thus ascribe to the reaction of dissolved O 2 with triplet excited states of rose bengal, the formation of singlet oxygen, and the subsequent reaction between 1 O 2 and probe compounds.