The aim of this study was the synthesis and physicochemical characterization of Ca(OH)(2) nanoparticles. The synthesis was carried out in ternary water-in-oil microemulsions where the oil phase was cyclohexane; two different microemulsion systems were investigated changing the nonionic surfactant from the tetraethylene-glycol-monododecyl ether (C12E4) to the pentaoxyethylene-glycol-nonyl-phenyl ether (Igepal-CO520). Phase diagrams of the two microemulsion systems were studied in the presence of high concentration of Ca2+ or OH- in the aqueous pool, allowing the determination of the optimum concentrations of the three components (surfactant, cyclohexane, CaCl2, and NaOH aqueous solution) employed to carry out the nanoparticles' synthesis. The microemulsions were characterized by Quasi-Elastic Dynamic Light Scattering (QELS) obtaining a linear relationship between the hydrodynamic diameter and the omega(o)-parameter both for the pure (NaOH and CaCl2) and the mixed systems. The nanoparticles' synthesis was performed in w/o microemulsions with different values of the omega(o)-parameter varying in the ranges 1.5-12 and 2-5 for the Igepal-CO520 and C12E4 systems, respectively. The particles, obtained were characterized by Transmission Electron Microscopy (TEM), X-ray Diffractometry (XRD), and Fourier Transform Infrared Microspectroscopy (FTIRM). The nanoparticles' size varied in the range 2-10 nm, and the average dimension depended on the omega(o)-parameter. The synthesized Ca(CH)2 nanoparticles were shown to be highly reactive against carbonatation by atmospheric CO2; therefore, the method was shown to be very effective also to prepare ultra-fine CaCO3 particles.