The oxidation of adrenaline (H2LH+) by molecular oxygen in the presence of catalytic amounts of VO2+ ions has been followed using a Clark-type oxygen electrode. The empirical rate law –d[O2]/dt=kobs.[O2]+p was obtained in which p is a small constant (only observable at low pH) and kobs. is given by the relationship (i), where [VO]T and [L]T are total initial concentrations of kobs.=A[VO]T[L]T// B[H+]2+C[L]T+D[L]T2 +E//[H+] +F(i) VO2+ and adrenaline respectively. It is demonstrated that this behaviour is consistent with the involvement of both VO2+ and V2+ species and furthermore that it also calls for the presence of a tris(adrenaline) species, V(LH)3+. A kinetically determined equilibrium constant for reaction (ii) is, VO(LH)2+ H2LH+⇌ V(LH)3++ H2O (ii) reported. The necessary proton– and metal–ligand equilibrium constants were obtained by pH- and 1H n.m.r.-titration techniques. All solutions were made up to a constant ionic strength (I= 0.100 mol dm–3) with KNO3 at 25.00 ± 0.02 °C.