The viscosity of a water-rich microemulsion phase composed of C(12)E(5), water, and decane has been measured. Attention has been focused on the concentration dependence of the viscosity along the emulsification failure boundary, where the microstructure of the system is that of oil-swollen spherical micelles. At high dilution, the concentration dependence of the relative viscosity is consistent with the behavior of a suspension of spheres, with an effective micellar size characterized by its hydrodynamic radius. At higher concentrations, where interactions are important, a good correlation with colloidal hard-sphere systems is obtained if the apparent micellar size is characterized by the hard-sphere radius. With increasing temperature away from the emulsification failure boundary, a significant increase in the viscosity is observed. The observed temperature dependence of the viscosity correlates with results obtained previously from H-2 NMR relaxation and self-diffusion experiments and is consistent with a micellar growth with increasing temperature.