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Water sorption in sulphonated polyimides with or without ionic block structure was analysed with Feng's new dual mode model. The effect of their molecular structure that determines the chain organisation in the solid materials was analysed by using the model parameters. The model parameters Cp and A' correspond to the sorbed water molecules on the first layer close to the ionic groups and on the subsequent layers, respectively. Based on these fitted physical parameters, the water sorption on the membranes with different counterions was studied and the hydration energy was proved to have much influence on the Cp values. The effect of the structure of the block and the random copolymers on the Cp and A' values was discussed and compared with that for the well-known Nafion® membranes. The large amount of sorbed water at high activities may induce a sufficiently large mobility of the polymer segments in the hydrophilic domains for material inflation, which leads to high A' values.