Quantitative effects of mater on the viscosity and rheology of magnesium-and sodium-neutralized dicarboxy-terminated polybutadiene in toluene are reported. A change in the water concentration from 50 to 500 ppm led to a viscosity decrease of 2 orders of magnitude for solutions near the gel concentration. Because of the hygroscopic nature of ionomer solutions, rheological experiments were conducted in a humidity-free environment. The dynamic measurements show that the viscosity seduction is related to a reduction in both the plateau modulus and the terminal relaxation time. The magnitude of shear-thickening decreases and the onset of shear-thickening shifts to higher shear rates with increasing water content.