The volume changes of polyaniline thick films (∼200 μm) upon redox switching in different acidic media were studied using voltammetric experiments with simultaneous microscopical observation. The area occupied by the polymer film in the microscope images was measured and used to compute the film volume and its variations. It is found that these films are considerably swelled by the electrolyte. During continuous potential cycling the film shows smooth volume changes, decreasing slightly at the beginning of the cycle, followed by a steeper increase in the last part of the positive scan; the volume changes show hysteresis and dependence on the electrolyte anion. When the polymer is completely reduced and relaxed the dimensional changes in the subsequent positive scan show, depending on the electrolyte, a small overall volume increase. During the negative scan the volume changes are similar in both cases, showing essentially two steps as the stationary positive scans. The results suggest that the volume changes are governed by several factors: ion and water exchange with the electrolyte, Coulombic repulsion between charged sites in the polymer backbone, anion–polymer interactions, and a structural change of the polymer backbone. With the aid of IR measurements, the influence of the different anions is interpreted in terms of their interaction with the polymer chain.