The formation enthalpy of palladium nanoparticles in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane microemulsions as a function of the water/AOT molar ratio (R = [water]/[AOT]) was measured by a calorimetric technique. The results indicate that at R < 10 the energetic state of the palladium nanoparticles compartmentalized within the reversed AOT micelles is signficantly different from that in bulk water. Effects due to the small size of the palladium nanoparticles and to interactions between nanoparticles and the water/AOT interface are discussed.