The UV absorbance and photochemical decomposition kinetics of hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. At higher pH borate/boric acid inhibits the photodecomposition of hydrogen peroxide (molar absorptivity and quantum yield of H2O2 and HO2−, (19.0±0.3) m−1 cm−1 and 1, and (237±7) m−1 cm−1 and 0.8±0.1, respectively). The results are consistent with the equilibrium formation of the anions monoperoxoborate, KBOOH=[H+][HOOB(OH)3−]/([B(OH)3][H2O2]), 2.0×10−8, R. Pizer, C. Tihal, Inorg. Chem. 1987, 26, 3639–3642, and monoperoxodiborate, KBOOB=[BOOB2−]/([B(OH)4−][HOOB(OH)3−]), 1.0±0.3 or 4.3±0.9, depending upon the conditions, with molar absorptivity, (19±1) m−1 cm−1 and (86±15) m−1 cm−1, respectively, and respective quantum yields, 1.1±0.1 and 0.04±0.04. The low quantum yield of monoperoxodiborate is discussed in terms of the slower diffusion apart of incipient .OB(OH)3− radicals than may be possible for .OH radicals, or a possible oxygen-bridged cyclic structure of the monoperoxodiborate.