The interaction between the electron donor o-phenylenediamine (o-phendn) and the pi-acceptor tetracyanoethylene (TCNE) has been investigated spectrophotometrically in dichloromethane at 25 degrees C. The results indicate that the reaction proceeds via two major steps. The formation of the 1:2 charge-transfer complex [(o-phendn)2, (TCNE)4] in which two o-phenylenediamine molecules undergoes slow N-substitution by one TCNE forming the final reaction product 2,2-di-o-aminoaniline-1,1-dicyanoethylene and 2HCN molecules. The reaction was studied using electronic, infrared, photometric titration and cyclic voltametric measurements. A general mechanism for the reactions is proposed.