Aqueous photochemistry of diazen-1-ium-1,2,2-triolate (Angeli's anion) and (Z)-1[N-(3-aminopropyl)-N-(3-aminopropyl)amino]diazen-1-ium-1,2-diolate (DPTA NONOate) has been investigated by laser kinetic spectroscopy. In neutral aqueous solutions, 266 nm photolysis of these diazeniumdiolates generates a unique spectrum of primary products including the ground-state triplet ((NO-)-N-3) and singlet ((HNO)-H-1) nitroxyl species and nitric oxide (NO center dot). Formation of these spectrophotometrically invisible products is revealed and quantitatively assayed by analyzing a complex set of their cross-reactions leading to the formation of colored intermediates, the N2O2 center dot- radical and N3O3- anion. The experimental design employed takes advantage of the extremely slow spin-forbidden protic equilibration between (NO-)-N-3 and (HNO)-H-1 and the vast difference in their reactivity toward NO center dot. To account for the kinetic data, a novel combination reaction, (NO-)-N-3 + (HNO)-H-1, is introduced, and its rate constant of 6.6 x 10(9) M-1 s(-1) is measured by competition with the reduction of methyl viologen by (NO-)-N-3. The latter reaction occurring with 2.1 x 10(9) M-1 s(-1) rate constant and leading to the stable, colored methyl viologen radical cation is useful for detection of (NO-)-N-3. The distributions of the primary photolysis products (Angeli's anion: 22% (NO-)-N-3, 58% (HNO)-H-1, and 20% NO center dot; DPTA NONOate: 3% (NO-)-N-3, 12% (HNO)-H-1, and 85% NO center dot) show that neither diazeniumdiolate is a highly selective photochemical generator of nitroxyl species or nitric oxide, although the selectivity of DPTA NONOate for NO center dot generation is clearly greater.