The volume changes of poly(2-methylaniline) upon redox cycling in different acidic media was studied using voltammetric experiments with simultaneous microscopic observation. These experiments were performed during continuous potential cycling and also when the polymer film was completely reduced and relaxed. The changes in the area occupied by the polymer film in the microscope images was taken as a measure of volume variations. During continuous potential cycling the films showed smooth, relatively low changes, showing hysteresis and dependence on the electrolyte anion. When the polymer was completely reduced and relaxed, its volume reached a minimum. The dimensional changes had a steep increase in the subsequent anodic scan, the overall change being about an order of magnitude higher than in continuous cycling, with little dependence on the electrolyte anion. During the cathodic scan the volume changes were similar to the stationary cycling experiments. These results suggest that the volume changes are due to two different processes, the faster one being proton and anion exchange with the electrolyte, both carrying solvent molecules. The other process is a structural change of the polymer backbone, giving higher volume changes but having, during the reduction step, a much lower rate.