The oxidative reaction between lanthanum trichloride and oxide ion was studied in the molten LiCl-KCl eutectic at 773 K by in-situ Raman spectroscopy using an yttria-stabilized zirconia electrode in conjunction with electrochemical methods. The formation of lanthanum oxychloride, LaOCl(s), was confirmed by studying the vibrational properties of crystalline LaOCl and X-ray diffraction analysis. The solubility product of LaOCl in the LiCl-KCl molten eutectic at 773 K is found to be pKsp(LaOCl) = 7.569 ± 0.100, by combining the results of Raman spectroscopy and potentiometric titration with the addition of barium oxide. The use of Raman spectroscopic quantitation of dissolved lanthanum ions in the system allows faster and more accurate determination of the stable phase of lanthanum as well as the solubility product of LaOCl compared with that measured by potentiometric titration only. Based on experimentally obtained pKsp and standard equilibrium potentials, the potential-pO2− phase diagram of lanthanum in the molten LiCl-KCl eutectic was reported.