Large transitions in viscosity are demonstrated for suspensions of environmentally sensitive gel microspheres upon application of external stimuli. Depending on the number concentration of microspheres, these suspensions may thicken to become semisolid, essentially undergoing a sol-gel transition. These reversible viscosity transitions are directly related to the volume transitions observed previously in the literature for individual environmentally sensitive gels. Temperature-sensitive poly-N-isopropylacrylamide microspheres were synthesized, and the effects of microsphere concentration on the temperature-induced viscosity transitions were measured. The viscosity of poly-N-isopropylacrylamide microsphere suspensions could also be increased by addition of sodium do decyl sulfate. Non-temperature-sensitive, commercially available polyacrylamide microspheres were used to demonstrate viscosity transitions as a function of solvent composition in acetone/water mixtures. Potential applications for these systems were suggested.