Peroxynitrite is a versatile and important biological oxidant that is produced from the reaction of nitric oxide and superoxide radicals. Two mechanisms have been proposed to rationalize oxidation reactions of peroxynitrite. One assumes that HO-ONO can homolize to form the hydroxyl radical and nitrogen dioxide, and that the hydroxyl radical is the proximate oxidant in peroxynitrite systems. The second argues this homolysis is too slow to occur at ordinary temperatures and suggests an excited species, HOONO*, is the proximate oxidant. If the radical mechanism is correct, then peroxynitrite should disappear more slowly in solvents of higher viscosity. This is true because for free radical initiators undergoing single-bond homolysis : (1) cage return is substantial and more of the cages would return to re-form HO-ONO as the viscosity of the medium increases; and (2) diffusion from the radical cage competes effectively with other cage processes. We have studied the disappearance of peroxynitrite at pH 5 and 7 in buffers with and without dioxane (as a control) or up to 30 wt % of the poly(ethylene glycol) (PEG) polymers, PEG 3350 and PEG 8000. These polyethers produce substantial changes in viscosity, raising the viscosity from about 0.89 to about 17 mPa . s. The rate constant for diffusion should decrease by about 10- to 20-fold as the viscosity increases in this interval, and the rate of diffusion from the solvent cage would be predicted to vary accordingly. However, at pH 5, where most of HOONO is undissociated, no change in the rate of disappearance of peroxynitrite is observed with increasing viscosity. At pH 7, a small increase in the observed rate constant is found, but it is likely due to the greater concentration of the undissociated HOONO in the ether-containing solvents resulting from a pK(a) shift. Thus, we conclude that the viscosity test does not support a free radical mechanism for the unimolecular decomposition of peroxynitrite.