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The electrochemical properties of poly (3,4-ethylenedioxythiophene) are studied using the bending beam method to detect volume changes during electrochemical transformations of the material. Thin films of poly (3,4-ethylenedioxythiophene) immersed in different supporting electrolytes first contract very rapidly and then expand on doping, while upon undoping they contract directly, or first expand and then contract, to their original positions. It is clearly observed that the oxidation or reduction of the polymer contains two steps, one due to a redox potential close to -0.5 V vs Ag/AgCl, and another potential around 0 V. We find that the volume changes cannot be understood as a simple consequence of ion transport but must be due to the structural change of the polymer between the different states. A hypothetical picture is that during the transition from the neutral to the polaron state, the polymer is slightly charged and thus contracted; on further doping to the bipolaron and to the metallic state, the coulomb repulsion between charged sites become stronger, and the polymer expands.

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