The electrochemistry of aryl diazonium salts is usually dominated by one-electron reduction, loss of dinitrogen, and the grafting of aryl radicals onto the electrode. In contrast, p-nitro- and p-cyanobenzene diazonium salts dissolved in mixed aqueous-acetonitrile solvents showed diffusion-limited, quasireversible, two-electron cyclic voltammetry. Voltammetric pH-dependence and spectrophotometric kinetic studies suggest that the basicity and low polarity of the aqueous solvent mixture has revealed the biologically-significant interconversion of diazohydroxide and diazene.