The surface roughness changes during redox-switching of poly(N-methylpyrrole)/poly(styrene sulfonate) (PNMP/PSS) of various thicknesses were investigated in aqueous perchlorate solutions of different cations, utilizing in situ scanning tunneling microscopy (STM). In situ STM studies show that both the surface roughness of PNMP/PSS and the roughness difference between doped and dedoped states become greater with the increase of film thicknesses, when the thickness is over a certain critical value (ca. 2 mu m). It was also found that identity of dopant cations can affect the surface roughness. While PNMP/PSS (4.0 mu m thick) reduced in LiClO4 produces a relatively smooth surface (R-a = 2.58 nm), PNMP/PSS reduced in KClO4 and CsClO4 exhibits R-a of 2.89 and 3.24 nm, respectively. This trend holds for an entire potential range examined, even when the film was fully oxidized to minimize the dopant cation involvement (R-a = 5.08, 6.24, and 7.79 not for Li+, K+, and Cs+, respectively). (c) 2005 The Electrochemical Society. All rights reserved.