The oxidation reaction of 2-aminophenol (OAP) to 2-aminophenoxazin-3-one (APX) initiated by 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) has been investigated in methanol at ambient temperature. The oxidation of OAP was followed by electronic spectroscopy and the rate constants were determined according to the rate law -d[OAP]/dt = k(obs) [OAP] [TEMPO]. The rate constant, activation enthalpy and entropy at 298 K are as follows: k(obs) (dm(3) mol(-1) s(-1)) := (1.49 +/- 0.02) x 10(-4), Ea = 18 +/- 5 kJ mol(-1), DeltaH(++) = 15 +/- 4 kJ mol(-1), DeltaS(++) = -82 +/- 17 J mol(-1) K-1. The results of oxidation of OAP show that the formation of 2-aminophenoxyl radical is the key step in the activation process of the substrate. (C) 2002 Elsevier Science B.V. All rights reserved.