The polarographic reduction of 4-nitro DPA in methanol-water mixtures of varying composition (30-80% v/v) in the pH range 1-12 was investigated. A single wave was observed in the reduction in these media at all pH values and solvent compositions. The number of electrons involved in the reduction was found to be six in all solutions indicating that 4-amino DPA is the final product. A mechanism consistent with these observations which involves the formation of 4-hydroxylaminodiphenylamine as an intermediate has been proposed. © 1978 Elsevier Sequoia S.A.