Photolysis of aqueous NO3- with lambda >= 195 nm is known to induce the formation of NO2- and O-2 as the only stable products. The mechanism of NO3- photolysis, however, is complex, and there is still uncertainty about the primary photoprocesses and subsequent reactions. This is, in part, due to photoisomerization of NO3- to ONOO- at lambda < 280 nm, followed by the formation of (OH)-O-center dot and (NO2)-N-center dot through the decomposition of ONOOH (p(Ka) = 6.5-6.8). Because of incomplete information concerning the mechanism of peroxynitrite (ONOOH/ONOO-) decomposition, previous studies were unable to account for all observations. In the present study aqueous nitrate solutions were photolyzed by monochromatic light in the range of 205-300 nm. It is shown that the main primary processes at this wavelength range are NO3 (hv) over right arrow center dot NO2 + O center dot- (reaction 1) and NO3- (hv) over right arrow ONOO- (reaction 2). Based on recent knowledge on the mechanisms of peroxynitrite decomposition and its reactions with reactive nitrogen and oxygen species, we determined (1) and (2) using different experimental approaches. Both quantum yields increase with decreasing the excitation wavelength, approaching Phi (1) 0.13 and Phi (2) 0.28 at 205 nm. It is also shown that the yield of nitrite increases with decreasing the excitation wavelength. The implications of these results on UV disinfection of drinking water are discussed.