The phase diagram, viscosity, adiabatic compressibility, optimum salinity, etc. for two microemulsion systems were studied. The systems were heptane and water with (a) Brij 35 + Sodium dodecyl sulfate and (b) Brij 35 + Tween 20 as surfactants. 1-Propanol was used as cosurfactant. The Frenkel-Eyring viscosity equation was used to calculate activation enthalpy of viscous flow. It was found that activation enthalpy was a function of the nature of dispersion medium. The free energy of Solubilization of oil in water was also calculated at the lower concentration region assuming ideality at infinite dilution. The free energy of solubilization was found to be a function of surfactant composition and the enthalpy more or less independent of composition.