A time-resolved optical (UV-vis) spectroscopic study of Roussin's black salt anion, Fe4S3(NO)(7)(-), revealed two separate intermediates, X and Y, following laser flash photolysis. Both intermediates react with nitric oxide with second-order kinetics to re-form the parent complex (k(NO)(X) = 1.3 x 10(7) M-1 s(-1); k(NO)(Y) = 7.0 x 10(5) M-1 S-1 in aqueous solutions). The shorter-lived intermediate X was observed in time resolved infrared spectroscopic studies. Isotopic labeling experiments involving the exchange of black salt nitrosyls with (NO2-)-N-15 or (NO)-N-15-O-18 were used to probe the correlation between nu(NO) bands and the anion structures. The identities of the intermediates are interpreted in terms of photolytic loss of chemically distinct nitrosyls found on the Fe4S3(NO)(7)(-) anion.