In this work, we report the preparation of the Sm2O3 nanoparticle precursors (Sm(OH)(3)) Via a simple W/O microemulsion process, in which microemulsions of cetyltrimethylammonium bromide (CrAB)/alkanol/1-octane/Sm(NO3)(3) aqueous solution were added into sodium hydroxide (NaOH) aqueous solutions. The Sm2O3 nanoparticles were then prepared by calcining the precursors at 900 degrees C. Particularly, Delta G(c -> i), which is the change in standard Gibbs free energy for transferring cosurfactant from the Continuous phase to the microemulsion interface and can be used to estimate the thermodynamic properties of microemulsions, was determined using the dilution method. The effects of alkanol carbon chain length (1-pentanol, 1-hexanol, 1-heptanol and 1-octanol) and the reaction temperatures (298, 308, 318 and 328 K) on both Delta G(c -> i) and Sm2O3 nanoparticle size have been investigated. Specifically, the Sm2O3 nanoparticle size, when calcined at 900 degrees C, was found to be mainly controlled by Delta G(c -> i), and was thereby affected by the thermodynamic properties of microemulsions. The obtained products were characterized by DSC-TGA. XRD, TEM and UV-Vis. The results showed that Delta G(c -> i) decreased with the increase in both the length of alkanol carbon chain and the reaction temperature, and the average size of Sm203 nanoparticles decreased as Delta G(c -> i) decreased. The effect of microemulsion thermodynamic properties on Sm2O3 nanoparticle size reported here can provide some insights in controllable preparation of other Fare earth oxide nanoparticles via the microemulsion route. (C) 2009 Elsevier Inc. All rights reserved.