We have measured the viscosity of water/AOT/isooctane microemulsions as a function of the droplet volume fraction at different temperatures for two values of the molar water to surfactant ratio w0:25 and 35. From the data we conclude that the microemulsion droplets behave like solid spheres, since (1) the low volume fraction dependence of the relative viscosity follows Einstein’s relation and (2) at sufficiently low temperatures the coefficients of the second- and third-order terms in the volume fraction expansion of the relative viscosity equal those found for latex and silica spheres. Furthermore, we find that the droplets aggregate reversibly at the higher temperatures. The enthalpies of binding of the observed clustering process were found to be 89 and 121 kJ/mol for w0 = 25 and 35, respectively, which is in good agreement with dielectric permittivity measurements on the same system.