Communication: Conjugated polymers such as polypyrrole (PPy) have potential use as artificial muscles or in microsystems such as valves for micro-fluid handling. One of the most important parameters in such uses is the magnitude of volume change during associated redox processes; however, until now estimates have varied greatly. Atomic force microscopy is reported here as allowing direct measurement of the in situ thickness change during oxidation and reduction of thin films of PPy doped with dodecylbenzenesulfonate.