The electrochemical oxidation of the ionophoric macrocycle N-phenylaza-15-crown-5 has been investigated in acetonitrile. For a 1 mmol dm(-3) solution of N-phenylaza-15-crown-5, cyclic voltammetry revealed that the mechanism followed that of N,N-dimethylaniline, the initial one-electron irreversible oxidation leading to dimerisation at the para position. Constant potential electrolysis in the presence of pyridine allowed stoichiometric electrogeneration of the dimer in solution; subsequent cyclic voltammetry showing two reversible one-electron redox waves (E(f) = 0.57 and 0.70 V vs. SSCE) to the dimer radical cation and quinoidal dication respectively. Cyclic voltammetric investigations revealed that both the N-phenylaza-15-crown-5 irreversible oxidation wave and the reversible N-phenylaza-15-crown-5 neutral dimer-to-radical cation redox wave were sensitive to the binding of sodium and magnesium guest cations at the macrocyclic coordinating sites. Thus shifts to more positive potentials were found on titrimetric addition of these cations, the higher charge:radius ratio of the magnesium cation guest producing the most dramatic effect.