The characteristics of the spectroscopic responses to membrane potential are examined for a series of dyes based on the 4-(p-aminostyryl)-1-pyridinium chromophore. An apparatus using an oxidized cholesterol hemispherical bilayer and phase-sensitive detection provides response spectra in either transmission or fluorescence excitation modes. All the probes with good binding properties display biphasic response spectra that are similar in both shape and magnitude. Detailed analysis of the response spectra allows all the previously discovered mechanisms for extrinsic potential sensitive molecular probes, which require a change in the probe's chemical environment, to be ruled out. The data are consistent with an electrochromic mechanism. Polarized fluorescence intensities from the membrane-bound probes indicate that the chromophore is optimally oriented for an electrochromic response.