The fast reaction of (NO)-N-center dot and O-2(center dot-) to give ONOO- has been extensively studied at irreversible conditions, but the reasons for the wide variations in observed forward rate constants (3.8 <= k(f) <= 20 x 10(9) M-1 s(-1)) remain unexplained. We characterized the diffusion-dependent aqueous (pH > 12) chemical equilibrium of the form (NO)-N-center dot + O-2(center dot-) = ONOO- with respect to its dependence on temperature, viscosity, and [ONOO-](eq) by determining [ONOO-](eq) and [(NO)-N-center dot](eq). The equilibrium forward reaction rate constant (k(f)(eq)) has negative activation energy, in contrast to that found under irreversible conditions. In contradiction to the law of mass action, we demonstrate that the equilibrium constant depends on ONOO- concentration. Therefore, a wide range of k(f)(eq) values could be derived (7.5-21 x 10(9) M-1 s(-1)). Of general interest, the variations in k(f) can thus be explained by its dependence on the distance between ONOO- particles (sites of generation of (NO)-N-center dot and O-2(center dot-)).