The polarographic reduction of Ni (II) in the presence of p-phenylenediamine (PPDA) has been studied in noncomplexing and in weakly complexing electrolytes. Catalytic prewaves, similar to those previously reported with o-phenylenediamine (OPDA), are observed only in the latter case. The reaction mechanism is believed to involve an aquated nickel-anion complex formed in the bulk of the solution and the adsorbed organic ligand. A good correlation is found between the prewave currents obtained with OPDA and Ni(II) in various electrolytes, and the rate of ligand exchange reported by others between corresponding nickel-anion complexes and a large organic anion. Experimental data for the PPDA and OPDA-Ni(II) systems agree qualitatively with the predictions of kinetic equations derived for an electrode reaction between a nickel-anion complex and an adsorbed ligand. Experimental data are also presented describing the polarographic results obtained with Ni(II) and isomers of the naphthalenediamines and with an inorganic ligand, N3-.