The formation enthalpy of gold nanoparticles in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane microemulsions as a function of the water/AOT molar ratio R(R = [water]/[AOT]) was measured by a calorimetric technique at various AOT concentrations. The size of An nanoparticles was determined by transmission electron microscopy. The results indicate that both the energetic state and the dimensions of the gold nanoparticles are influenced by the radius and the concentration of the reversed AOT micelles. Analysis of the decay of the calorimetric signal provides evidence of some interesting features of the growing process of the Au nanoparticles in water/AOT/n-heptane microemulsions.