A new dual-mode sorption model (NDMS) was applied to sigmoid-shaped isotherms of water vapor sorption in composites of sulfonated or nonsulfonated polysulfones with a sulfonic-modified laponite clay and in blends of sulfonated poly(ether ether ketone) with sulfonated polyether sulfone cardo, respectively. The three NDMS parameters, C-p, A', and k' can be correlated to the amount of water molecules sorbed in the first hydration shell, the subsequent sorption on the sulfonic-sites and the tendency for water molecules to form clusters at very high water activities, respectively. The fitted values were used to study the relationship between sorption behaviors and component structure or organization in the composites. The analysis of the sorption-desorption cycles shows that the sorption hysteresis increases with the ionomer chain rigidity. An analytical expression for the mean cluster size (MCS) as a function of activity was deduced and validated by correlating to sorption kinetics. It has been shown that MCS increases with chain flexibility at high water activities.