The dimeric dioximatomanganese(II) complex [Mn 2 (HL) 2 ](BPh 4 ) 2 (1) (where H 2 L is [HONC(CH 3 )C(CH 3 )NCH 2 CH 2 ] 2 NH) dissociates to the monomer [Mn(HL)] + (2) upon dissolution in MeOH. Complex 2 is a functional phenoxazinone synthase model catalyzing the base-assisted oxidative dimerization of 2-aminophenol (H 3 ap) to 2-amino-3H-phenoxazine-3-one (apx) by dioxygen under ambient conditions via an o-benzoquinone monoimine (bqmi) intermediate, which, however, cannot be directly detected due to its high reactivity. If H 3 ap is replaced by 2-anilino-4,6-di-tert-butylphenol (3), a structural analog where the required positions are blocked, oxidation occurs without dimerization, affording the corresponding N-substituted o-quinone monoimine. Free radical intermediates were detected by ESR spectroscopy in both reactions. The kinetics of the catalytic oxidation of H 3 ap was studied spectrophotometrically and the corresponding rate equation established. It is consistent with a mechanism involving the formation of a catalyst-H 2 apO 2 ternary complex, which decomposes in the rate-determining step, generating a semiquinone monoimine undergoing noncatalytic oxidative dimerization to apx.