Copolymers of N-vinylcarbazole (NVC) and the quarternary salt of N,N-dimethylaminoethyl methacrylate with ethyl iodide (DMAEMA.EI) were prepared by free-radical copolymerization in dimethyl sulfoxide at 70 degrees C using alpha,alpha＇-azobisisobutyronitrile (AIBN) as an initiator. The reactivity ratios of both monomers were determined according to the general copolymerization equation using the extended Kelen-Tudos method and found to be r(1) = 0.12 (+/-0.04) and r(2) = 0.84 (+/-0.08). Copolymers were characterized by viscosity measurements and thermogravimetric analysis (TGA). Copolymers containing DMAEMA.EI higher than 60 mol % were water-soluble.