UV-vis spectroscopy was utilized to measure the decomposition of aqueous HNO3 solutions above 300 degrees C, in some cases with added NaOH, H2O2, and/or NaNO2, to form NO2, HNO2, NO, N2O, and oxygen. Individual hands corresponding to HNO2 and NO2 were deconvoluted from thr spectra up to 400 degrees C. With constrained nonlinear optimization, areas of these bands were used to determine extinction coefficients of NO2 and HNO2 and equilibrium constants for the reactions: HNO3 right arrow over left arrow H+ + NO3-; 2HNO(3) right arrow over left arrow H2O + 2NO(2) + (1/2)O-2; 2NO(2) + H2O right arrow over left arrow HNO3 + HNO2: 2HNO(2) right arrow over left arrow H2O + 2NO + (1/2)O-2; 2NO right arrow over left arrow N2O + (1/2)O-2; (Na+)(NO3-) right arrow over left arrow Na+ + NO3-.