Effects of the polycation structure and of the solvent nature and composition on the viscometric behaviour of some cationic polyelectrolytes with (N,N-dimethyl-2-hydroxypropylene ammonium chloride) units in the main chain and non-polar side groups (hexyloxypropyl and decyloxypropyl) were investigated. The solution viscosity of polyelectrolytes was determined over a wide concentration range, in saltless aqueous solution. The experimental data were plotted in the terms of Fuoss and Fedors equations. Viscosity of the polycation PCA(5)D(1), was measured also in the mixed solvents: water/methanol and water/acetone. Polyelectrolyte behaviour was observed for PCA(5)D(1) in the mixtures of water/methanol for all composition employed and in mixtures of water/acetone with the acetone content as high as 60 vol%. The intrinsic viscosity, [eta] decreased and the overlap concentration, C*, increased with increasing organic solvent content in the mixed solvents. (C) 2001 Elsevier Science Ltd. All rights reserved.