A new procedure for measuring electrochemical impedance spectra directly during potential cycling is presented. The method consists in taking a series of impedance-potential curves at different potential modulation frequencies. Then the impedance values at desired electrode potentials are sampled, and corresponding dynamic impedance spectra are reconstructed. The method was applied to a thin-film poly-3-methylthiophene Pt (P3MTPt) electrode in an acetonitrile solution. The results suggested that undoping of the polymer at negative potentials gave rise to development of a space-charge region within the film that hindered the polymer doping process unless flat-band conditions were reached. Such behavior could account for a well-known memory effect observed in conducting polymers. Moreover, our data provide further evidence that illumination of a P3MT electrode at negative potentials results in photoelectrochemical undoping of the polymer.