A two-step method was developed to synthesize sulfobetaine copolymers with different comonomer units in the main chain. The alternating copolymers, poly(N-vinyl-2-pyrrolidone-co-phthalimidoacrylate) and poly(styrene-co-phthalimidoacrylate) were modified with 3-dimethylaminopropylamine to yield copolymers containing tertiary amine groups in the side chains. They were then quaternized with 1,3-propanesultone. The aqueous solution behaviour of the zwitterionic copolymers depended on the structure of the backbone. Thus, the copolymer derived from the N-vinyl-2-pyrrolidone-based precursor dissolved only in aqueous salt solutions. The reduced viscosities of these solutions were insensitive to the polymer concentration. Increasing the salt concentration and "softness" of the salt ions slightly increased the intrinsic viscosity. The results suggest the important role of the intramolecular interactions in the solution behaviour of this sulfobetaine copolymer. In contrast, the sulfobetaine copolymer containing styrene units in the main chain formed micellar solutions whose intrinsic viscosity decreased on salt addition.