Carbon kinetic isotope effects (KIEs) at natural abundances during photolysis of Fe3+-oxalato, malonato, and succinato complexes in aqueous solution were studied to identify the C-C bond cleaving mechanism of Fe3+-oxalato complexes under sunlight irradiation. Observed overall KlEs were 5.9 parts per thousand,11.5 parts per thousand, and 8.4 parts per thousand, respectively. This variation is inconsistent with secondary carbon KIEs for the Fe-O bond cleavage, but consistent with primary carbon KlEs for sequential cleavage of Fe-O and C-C bonds. Position-specific probability of C-13 content estimated KIEs of 5.9 parts per thousand, 17.2 parts per thousand, and 17 parts per thousand for C-12-C-13 bond cleavage, respectively, indicating the different KlEs for carboxyl-carboxyl and methyl-carboxyl cleavage. (C) 2016 Elsevier B.V. All rights reserved.