The guanidine moiety of arginine is involved in the active sites of a variety of enzymes, such as nitric oxide synthase (NOS) and NiFe hydrogenase. In this paper we aim to investigate the effects of a metal center on the oxidation of guanidine, which should provide an interesting comparison with the biological aerobic oxidation of arginine catalyzed by NOS. We studied the oxidation of an osmium(III) guanidine complex, mer-[Os(L){N(H)C(NH2)(2)}(CN)(3)](-), (OsG, HL = 2-(2-hydroxyphenyl)benzoxazole) by m-chloroperbenzoic acid (m-CPBA), which is potentially an O atom transfer reagent, and by (NH4)(2)[Ce-IV(NO3)(6)], which is a one-electron oxidant. With m-CPBA, mer-[Os(NO)(L)(CN)(3)]- (mer-OsNO) is the product, while with CeIV, mer-[OsVI(N)(L)(CN)(3)]- (mer-OsN) is formed instead. The crystal structures of mer-OsNO and mer-OsN were determined by X-ray crystallography. The mechanisms for the oxidation of OsG by m-CPBA and CeIV are proposed.