The rheology of solutions of a cationic cellulose ether (JR400) and either sodium poly(2-acrylamido-2-methylpropanesulfonate) (PAMPS) or a copolymer of sodium 2-acrylamido-2-methylpropanesulfonate and N-n-dodecylmethacrylamide (PAMPS-Dod) has been examined over a wide composition range in the semidilute regime (10 g L-1) by steady shear and time-dependent steady shear measurements. Miscible combinations of pairs of oppositely charged polyelectrolytes form in two composition domains: (1) a polyanion-poor region which exhibits shear thinning characteristics and (2) a polyanion-rich region which presents Newtonian behavior at low shear rates and, as the shear rate is increased, passes through a shear-thickening region before exhibiting a sharp decrease in viscosity. The latter fluids exhibit significant thixotropy, which is shown to depend on the delay time between measurements and on the composition of the fluids. Models based on electrostatically driven interactions between two oppositely charged polyelectrolytes poorly matched in terms of size, rigidity, and charge density are proposed to account for the unusual rheological behavior of the polyelectrolyte complexes under study.