The photolysis of nitrate, NO3-, in D2O solution has been investigated by femtosecond infrared spectroscopy. accordance with previous investigations, we observe that the peroxynitrite ion, ONOO-, is the dominant photochemical product following the excitation of nitrate at 200 nm. Moreover, we are able to identify the cis/trans isomers of peroxynitrite and the dynamics of their formation in solution. We observe that the trans-ONOO- isomer is formed directly and solely from the excited NO3- ion within the first two picoseconds after excitation. Subsequently, about half of the trans-ONOO- isomerizes to cis-ONOO- in 25 ps; thereafter, the ratio between the two isomers remains constant for the 300 ps duration of the experiment. The observed vibrational frequencies of the terminal O=N bonds are at 1515 and 1580 cm(-1) for trans- and cis-peroxynitrite, respectively. The detailed analysis of the infrared bands of cis- and trans-peroxynitrite is facilitated by electronic structure calculations on the conformers in a cluster of I I D2O molecules and by steady-state infrared spectroscopy of ONOO- in D2O, In addition to the formation of ONOO-, the experiments also reveal a slow similar to 50 ps formation of NO2 following the photolysis of nitrate.