Steady-state voltammetry and chronoamperometry at microelectrodes were used to study mass transport properties of temperature sensitive poly(N-isopropylacrylamide-co-acrylic acid), NIPA-AA, hydrogels. 1,1 ' -Ferrocenedimethylanol, Fc(MeOH)(2), and 2,2,6,6-tehamethyl-1-piperidinyloxy, TEMPO, were used as electroactive probe molecules. The activation energy of diffusion of Fc(MeOH)2 in aqueous solutions and in NIPA-AA hydrogels was found to be in the range of 17-19 kJ/mol, which suggests that the local microscopic viscosity does not change significantly because of the gelation process, although the macroscopic viscosity of the gels is extremely large. It was found that the diffusion coefficients of Fc(MeOH)2 and TEMPO in NIPA-AA hydrogels in their swollen state are approximately 20%-50% smaller than those in aqueous solutions, and that the diffusion coefficient of probe molecules in these gels is inversely proportional to the concentration of copolymer in the hydrogels. The "obstruction effect" and "hydration effect" were used to explain this phenomenon, and experimental results were compared with predictions of the model.