The surface segregation of cations in a poly(styrene-ran-methacrylic acid) ionomer fully neutralized with Cs was demonstrated using Rutherford backscattering spectrometry (RBS), scanning force microscopy (SFM), and scanning transmission electron microscopy (STEM). Whereas spin-cast films and those annealed below similar to 120 degrees C exhibit a uniform distribution of Cs, a surface excess of Cs was observed for films annealed at higher temperatures. At long times (> 30 h) and high temperatures (> 145 degrees C), the surface concentration of Cs approached a constant value of two-thirds of the total Cs in the film. Although Cs-rich vesicular aggregates (similar to 8-85 nm diameter) were observed in all films, the Surface excess of Cs coincided with nanometer-sized features on the surface. Based on these results, a mechanism was proposed that accounts for cation mobility and a driving force for surface segregation. At elevated temperatures, Cs ions initially in cation-acid lone pairs are solubilized by favorable cation-pi interactions facilitated by styrene monomers. Above similar to 120 degrees C, these solubilized cations are sufficiently mobile to diffuse. The driving force to the surface arises from the concentration gradient established when Cs at the surface scavenges Cl from the environment to form CsCl. In the poly styrene-based ionomers, surface segregation is not observed if either the cation mobility is reduced by using a divalent cation or the driving force for Surface segregation is removed by eliminating atmospheric Cl.