illustrated in Figure 2. Here, rates of ON02-decay are compared under identical medium conditions (200 mM phosphate, pH 7.6) for 500 µ ON02~ with no added carbonate (trace 2), with 2.5 mM HCO3-under near-equilibrium conditions (~ 75 µ C02, trace 3), and with~ 2.25 mM C02 plus 0. 25 mM HCO3-(trace 4). The rate of reaction increased proportionately with the C02 concentration, establishing that ON02-and C02 comprise a reactant pair. The possibility that 0N02H and HCO3-also comprise a reactant pair was investigated by a second type of pH-jump experiment in which alkaline solutions containing both CO32-and ON02” ions were mixed with acidic solutions containing only buffer to give neutral solutions that were initially devoid of C02. Rapid catalysis of ON02-decay was not observed under these conditions (Figure 2, trace 1). Kinetic analyses indicated that the rate of this reaction was controlled by the rates of HCO3-dehydration and spontaneous isomerization of 0N02-. 23 Thus, direct reaction between the species HCO3-and ON02H must be slower than these processes. This experiment was possible because CO32-was also found to be unreactive toward ON02-, 1. e., addition of carbonate to alkaline (pH> 11) solutions of ON02-did not accelerate its very slow intrinsic rate of decomposition. Additional evidence indicating that C02 and ON02~ comprise a reactant pair, but ON02H and HCO3-do not, is that biphasic kinetics were always observed when solutions containing excess HCO3-, but limiting C02, were mixed with solutions of ON02-(data not shown). The breakpoints of these kinetic curves corresponded to the amount of C02 initially present in the solutions; they indicate that when exposed to ONQ2-the existing C02 is rapidly consumed,