Pyrrole, thiophene, and a mixture of the two monomers were electrochemically polymerized to investigate polymerization rates and the morphology change of the polymer matrix, and to improve the aging and cyclic voltammetric behaviors of the polymers. Thiophene was polymerized on a smooth surface of Pt electrode by two steps. The first step was controlled by electron transfer at the electrical double layer and the other by diffusion of the monomer reacting on the immobilized layer consisting of the precoated thiophene polymer. The electropolymerization rate of the second step was 1.85 x 10(-4) cm(3) mol(-1) s(-1), which is faster by 8.63 x 10(2) times than the first step. Some supporting electrolytes such as KPF6, LiClO4, TBAP, and TBABF(4) were employed in the polymerization reaction to see the effects of dopant anions on the polymerization rate, and KPF6 was the fastest one at 2.41 x 10(-6) cm s(-1). However, owing to its sensitivity to oxygen, LiClO4 was used for the polymerization that is fairly stable in air and the same rate as KPF6. For the competitive polymerization reaction of the two monomers the rate of thiophene was found to be about 11 times slower than that of thiophene alone. When the starting concentration of the thiophene monomer was higher than pyrrole by five times, its portion in the composite polymer was found to be only 8-10%. However, this level gave desirable results in terms of redox properties and aging. The resistance against aging was explained by the morphology change, which came from great shrinking of its porosity.