Solution-processable polyelectrolyte complexes (PECs) were fabricated from sulfonated carboxymethyl cellulose (SCMC) and chitosan (CS) by two methods. Two states of sulfate groups (SO₃⁻) in PECs, i.e., complexed and free sulfate groups, were distinguished and shown to play important roles in determining mechanical properties and separation performances of PEC membranes (PECMs). PECMs containing different states of SO₃⁻ were subjected to pervaporation dehydration, and their hydrophilic properties and swelling behaviors were thoroughly discussed. It was found that the tensile strength of PECMs could be effectively enhanced by increasing the ratio of complexed SO₃⁻ groups, while the hydration and flexibility were modulated by free SO₃⁻ groups. In the dehydration of water–ethanol mixtures, both the flux and selectivity of PECMs were simultaneously improved through the incorporation of SO₃⁻ groups, whereby the improved pervaporation performance was mainly attributed to the free SO₃⁻ groups.