We have investigated the effects of film thickness and composition on the rate of swelling and capacitance response to pH changes for a thin copolymer film containing predominately polymethylene (PM) with randomly distributed carboxylic acid chains (denoted as PM-CO2H). These films are prepared onto gold substrates by surface-catalyzed polymerization of diazomethane and ethyl diazoacetate and subsequent hydrolysis. We used electrochemical impedance spectroscopy (EIS) at fixed frequency to measure film capacitance during a step change of pH. As water permeates into the film, it increases the dielectric constant of the film and enhances the capacitance. At a 1% - 3% molar acid content, a 90 nm copolymer film exhibits a 100-fold, reversible change in its capacitance over 2 – 3 pH units at 100 Hz. For the 1% acid film, its rate of protonation is ~20 times faster than its rate of ionization due to the more gradual permeation through the initially hydrophobic film during ionization and the rapid expulsion of water and ions once the film reverts to the hydrophobic state during protonation. However, films with 2-3% acid content film exhibit similar protonational and ionizational rates. These films can change from the uncharged to the charged states within 5 seconds. A 2% molar acid film with a thickness of 300 nm shows a similar protonational rate to the 2% acid film at 90 nm, but its ionizational rate is 8 times slower than its protonation rate, providing insight on the mechanism of water/ion transport in swelling/deswelling. A large, fast, and reversible pH response for sensor applications can be obtained by carefully selecting the film thickness and composition.