The water vapor uptake of sulfonated polyimides (SP) was investigated by electronic microbalance (IGA, Hiden) from 15 to 55 degreesC. The sigmoidal isotherms obtained (BET II type) are considered as dual sorption (concave part) plus clustering (convex part) and are fitted with good agreement by Park's equation, Zimm-Lundberg's method is used to study the clustering process of water molecules limit clustering activity, alpha*, and the number of molecules per cluster are estimated. To obtain a better understanding of polymer structure and isotherm analysis, H+ (counter-ions of sulfonic groups) were replaced by ions with a smaller hydration shell (Cs+ and EDAH(+)). Comparison of the three isotherms shows no significant difference in the water affinity of the cations. This is attributed to a partial control of the sorption by microcavities existing in the membrane.