The viscosity as a function of concentration for xanthan gum in both salt-free solution and in 50 mM NaCl is measured and compared with a scaling theory for polyelectrolytes. In general, the zero shear rate viscosity and the degree of shear thinning increase with polymer concentration. In addition, shear thinning was observed in the dilute regime in both solvents. In salt-free solution, four concentration regimes of viscosity scaling and three associated critical concentrations were observed (c* approximate to 70 ppm, c(e) approximate to 400 ppm, and C-D approximate to 2000 ppm). In salt solution, only three concentration regimes and two critical concentrations were observed (c* approximate to 200 pprn and c(e) approximate to 800 ppm). In the presence of salt, the polymer chain structure collapses and occupies much less space resulting in higher values of the critical concentrations. The observed viscosity-concentration scaling is in very good agreement with theory in the semidilute unentangled and semidilute entangled regimes in both salt-free and 50 mM NaCl solution. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 4076-4084, 2009