The preparation of poly(N-isopropylacrylamide-co-acrylic acid) ionomers with different desirable molar masses and ionic contents has accomplished by using a benzene/ethanol mixture as polymerisation medium, wherein benzene and ethanol acted as solvent and chain transfer agent, respectively. The volume ratio of benzene to ethanol in the mixture could be used to control the molar mass of the ionomer in a broad range from similar to 10(4) to similar to 5 x 10(6) g mol(-1). The ionic content was less affected by the solvent composition, but regulated by the monomer feed ratio. The ionomers were characterised by a combination of static and dynamic laser light scattering. The molar mass distributions of the ionomers were much narrower than those expected from free-radical polymerisation. The light scattering results revealed that the translational diffusion coefficient (D) scaled to the molar mass (M) as D (cm(2) s(-1)) = 3.56 x 10(-4) M-0.61.