The polypyrrole films containing perchlorate were electrochemically synthesized and the bending and recovery motion of the film obtained have been investigated. The principle of motion was based on a reversible sorption of water vapour on one side of the film, causing an expansion of the film surface. On the basis of this phenomenon, we have devised a polypyrrole rotor driven by sorption of water vapour, producing a continuous rotation at a speed of 21 cm min(-1).