Gravimetrie analysis was performed during a step change in relative humidity for a seriss of sulfonated polymers to determine their dynamic water uptake characteristics. The mass change was recorded as a function of time for both stiff-backboned and flexible-backboned polymers when the humidity was instantaneously increased from 40 to 50 %. The observed water uptake profiles were distinctly different based on the inferred stiffness of the sulfonated polymer backbone. A sulfonated poly(sulfone)-based membrane had a slower mass uptake than more flexible Nafion with time constants of 307 s and 150 s for the sulfonated poly(sulfone) and NRE212, respectively. The dynamic water absorption properties were measured for a series of sulfonated poly(styrene)-b- poly(vinylidene fluoride)-b-sulfonated poly(styrene) membranes. The backbone flexibility of these polymers was intermediate to that of the poly(sulfone) and Nafion membranes and could be tuned based on the molecular weight of the poly(styrene) blocks and sulfonation level.