Copolymerization of 2,5-diaminobenzensulfonic acid (DABSA) and aniline (AN) was performed electrochemically by cyclic voltammetry on IrO2-coated titanium electrodes in 1M HCl. The copolymerization rate is strongly dependent on the amount of DABSA in the comonomer feed. The compositions of the copolymeric films were determined by x-ray photoelectron spectroscopy and were controlled by changing comonomer feed ratios. The copolymer compositions can be expressed in terms of comonomer feed ratios by a simple copolymer model showing that the reactivity ratio of aniline increases by increasing the amount of DABSA in the comonomer feed. The composite film of DABSA-AN copolymer with waterborne polyurethane was prepared and its conductivity was measured for comparing the effect of DABSA in the comonomer feed. Scanning electron microscope (SEM) photographs show that there exist clear differences in morphology of the polymer films at various DABSA concentrations.