The photolysis of the HNO-myoglobin adduct has been followed by transient absorption experiments down to the nanosecond time scale. A unique transient assignable to generation of an Fe-III/HNO- geminate pair is observed; the transient loss of this absorbance trace, assignable to the recombination, is fit to a single-exponential decay with tau = 7.0 (+/- 0.5) x 10(-5) sec. Residual absorbance after the microsecond recombination indicates a small percentage escape of HNO-into solution during pulsed photolysis experiments. Thermal decomposition and bulk photolysis also generate metMb products; analysis of the reaction mixture after photolysis identified low yields of H2NOH, suggesting multiple secondary reactions of the released aminoxyl radical. Photolysis of N-15-labeled HNO-Mb showed only the presence of (15)N2O as gaseous product. Both the production of metMb and the microsecond recombination time scale suggest ferric character to the adduct, that is, (HNO-)-Fe(III)Mb, analogous to similar characterizations of the isoelectronic species, oxyMb as (O-2(-))-Fe(III)Mb.