Abstract: The effect of hydration on the Fourier Transform IR spectra of naphthenate anion was observed during the process of microemulsion formation by adding water gradually to anhydrous sodium naphthenate prepared by refluxing naphthenic acid-kerosine solution with metallic sodium. Hydration occurs both on the carboxylate anion and on the carbonyl group of the neutral carboxylic acid.The thermodynamic stability of microemulsion formed was explained by the large amount of energy liberated during the hydration of both cation and anion of sodium naphthenate.The properties of aqueous solutions of electrolytes can be advantageously studied by this microemulsion technique because it provides a very concentrated aqueous solution of electrolytes equivalent to more than 1000 molal in the usual concentration scale, so that many phenomena, otherwise not observable, could be studied in this way.