The oxidation reaction of o-phenylenediamine (PDA) to 2,3-diaminophenazine (DAP) initiated by 2,2,6,6-tetramethyl-1-pipetidinyloxyl (TEMPO) has been investigated in methanol at ambient temperature. The oxidation of PDA was followed by electronic spectroscopy and the rate constants were determined according to the rate law -d[PDA]/dt = k(obs)[PDA] [TEMPO]. The rate constant, activation enthalpy and entropy at 323 K are as follows: k(obs) (dm(3) mol(-1) s(-1)) = (3.60+/-0.13) x 10(-6), E-a (kJ mol(-1)) = 76+/-11, DeltaH(double dagger) (kJ mol(-1)) = 74 +/- 10, DeltaS(double dagger) (J mol(-1) K-1) = -129+/-31.