Free-standing films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) with various PSS contents were newly prepared by casting water dispersion of the PEDOT/PSS colloidal particles in the presence of an extra PSS. Electrical conductivity, morphology, water vapor sorption, and electro-active polymer actuating behavior of the resulting films were investigated by means of four-point method, atomic force microscope (AFM), sorption isotherm, and electromechanical analyses. The maximum contraction of the film by application of an electric field increased with increasing both PSS content and relative humidity (RH), where the value attained 7% at 70% RH for the film with 93% of PSS. Since the isothermal sorption curve of the film was less dependent on the PSS content, the significant increase of the film contraction was explained by two mechanisms: (i) the extra PSS prevented from hydrogen bonding between adjacent PEDOT/PSS particles that suppressed dimensional changes of the film; and (ii) the higher the RH, the larger the degree of water vapor sorption, which led to the large film contraction by desorption of water vapor via Joule heating. On the basis of this phenomenon linear actuators utilizing PEDOT/PSS films were successfully developed and applied to leverage actuator and Braille cell. (C) 2012 Elsevier Ltd. All rights reserved.