The molar enthalpies of precipitation of calcium monohydrogen phosphate (CaHPO4) in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane microemulsions were measured at 25 degrees C by a calorimetric technique as a function of the molar ratio R (R = [water]/[AOT]) at a fixed AOT concentration. Calorimetric data indicate the formation of CaHPO4 nanoparticles encapsulated in the aqueous core of the AOT reversed micelles displaying a slow growing process with time. Their energetic state is initially different from that in bulk water approaching the value in water at longer rimes. The observed growing process results to be completely inhibited by the presence of small amounts of bis(2-ethylhexyl) phosphate acid (DEPA) leading to the formation of stable nanosized calcium monohydrogen phosphate: particles coated by an oriented monolayer of chemically bonded DEPA molecules. After evaporation of the volatile components of the microemulsions, it was observed by transmission electron microscopy that CaHPO4 nanoparticles embedded in the surfactant matrix are still present.