The decay mechanism of NO3 center dot has been determined through a combination of experiment and calculation for 7 mol dm(-3) solutions of deaerated aqueous LiNO3 and HNO3, in the absence and presence of hydrazine (N2H4, N2H5+, and N-2,H-6(2+)). In the absence of hydrazine, the predominant NO3 center dot decay pathways are strongly dependent upon the pH of the solution. For neat, neutral pH LiNO3 solutions (7 mol dm 3), NO3 center dot produced by the pulse is fully consumed within 160 mu s by OH center dot (37%), H2O (29%), NO2- (17%), and NO2 (17%). For acidic HNO3 solutions (7 mol dm-3), radiolytically produced NO3 center dot is predominantly consumed within 1 ms by HNO2 (15%) and NO2 (80%). Intervening formulations exhibit the mechanistic transition from neat LiNO3 to neat HNO3. In highly acidic nitric acid solution, hydrazine exists mainly as N2H5+ and N2H62+, both of which rapidly consume NO3 center dot in addition to other decay mechanisms, with rate constants of 2.9 (+/- 0.9) X 10(7) and 1.3 (+/- 0.3) X 10(6) dm(3) mol(-1) s(-1), respectively.