Nitrosylmyoglobin, MbFe(II)NO, was found to be oxidized by [Fe(CN)6](3-) and HClO/ClO(-), but not by the semistable radical nitrosodisulfonate (anion of Frémy's salt) or NO2 (-) at ambient temperature in aqueous solution with pH 7.0. The oxidation by HClO/ClO(-) was significantly faster than that by [Fe(CN)6](3-). With excess [Fe(CN)6](3-), MbFe(II)NO was oxidized to metmyoglobin, MbFe(III), in a second-order reaction with k 2 = 1.67 ± 0.10 M(-1) s(-1) at 288 K without detectable intermediates as determined by stopped-flow spectroscopy. The activation parameters were ΔH (‡) = 43 ± 2 kJ mol(-1) and ΔS (‡) = -93 ± 9 J(-1) K(-1) mol(-1). Outer-sphere electron-transfer to [Fe(CN)6](3-) was assigned as rate determining rather than NO dissociation from iron(II) followed by electron transfer. Outer-sphere electron transfer from MbFe(II)NO to certain moderate oxidizing agents may thus have a role in labilizing NO association slowly through oxidation of iron(II) to iron(III). In contrast, hypochlorite oxidizes MbFe(II)NO much faster in a complex sequence of processes involving a rate-determining second-order (unidentified) reaction with k 2 = 2.6 ± 0.3 × 10(3) M(-1) s(-1) at 288 K and possibly involving protein degradation.