Electrochemical polymerization of aniline was conducted in constant current mode at different pH ranging from 0.2 to 3.7 in an ITO glass electrode-quartz cell assembly for in situ monitoring of the changes in potential, amount of charge passed through, and u.v.-visible absorption (u.v.-Vis) spectra during polymerization. u.v.-Vis spectra of product polyaniline (PAn) films were determined in different pH solutions to evaluate the apparent acid dissociation constants (pKa)(app) from plots of log{(Abs(HAX) - Abs(obs))/(Abs(obs) - Abs(A)) versus pH, where Abs(obs), Abs(HAX), and Abs(A) are the observed absorption strength and those of the fully protonated and deprotonated species, respectively, for a given wavelength light. In PAn films prepared at low pH ( < 1.0), emeraldine form PAn chains with relatively long average conjugation length but broad distribution were produced, while in those prepared at high pH ( > 1.5), PAn chains with short conjugation length were produced and later deteriorated as the polymerization proceeded, because the polymerization potential eventually exceeded a critical value ( + 0.8 V) beyond which degradation of produced PAn chains took place.