The heat of water dissolution in binary mixtures of Aerosol OT (AOT) and heptane (Hp) leading to increasing mole ratios of water and AOT in the final microemulsion solution has been calorimetrically determined. The overall heat of solution has been found to be endothermic. On the basis of several heat involving processes, a rational analysis of the water dissolution phenomenon has been made which has helped to evaluate the enthalpy of water penetration. In the presence of NaCl and NaC the overall heat of solution (Q(sol)) has decreased. The specific heats of the resulting solutions have supported the formation of an initial open structure followed by clustering of microdroplets at higher mole ratios of water and AOT.