Temperature dependence of water vapor sorption and electro-active polymer actuating behavior of free-standing films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) was investigated by means of sorption isotherm and electromechanical analyses. The non-porous PEDOT/PSS film, having a specific surface area of 0.13 m 2 g -1 , sorbed water vapor of 1080 cm 3 (STP) g -1 , corresponding to 87 wt%, at relative water vapor pressure of 0.95. A temperature rise from 25 β’ C to 40 β’ C lowered sorption degree, indicative of an exothermic process, where isosteric heat of sorption decreased with increasing water vapor sorption and the value reached 43.9 kJ mol -1 , being consistent with the heat of water condensation (44 kJ mol -1 ). Upon application of 10 V, the film underwent contraction of 2.46% at 5 β’ C caused by desorption of water vapor due to Joule heating, which slightly decreased to 2.10% at 45 β’ C. The speed of contraction was one order of magnitude faster than that of expansion and less dependent on the temperature since water vapor sorbed in the film were forced to desorb by Joule heating. In contrast, the higher the temperature the faster the film expansion because diffusion coefficient increased as the temperature became higher.