Abstract: The solubilities and the refraction indices of the saturated solutions in the titled ternary system have been investigated isothermally at 0℃ and 30℃.At 0℃, both the isotherms of solubilities and refraction indices of the system consist of,three branches corresponding to the solid phases, CH₃CONH₂, LaCl₃·4CH₃ CONH₂·5H₂O and LaCl₃·7H₂O, respectively. The compositions of the two eutonic points are 52.47% (by wt.) CH₃CONH₂, 21.77,% LaCl₃ and 9.46% CH₃CONH₂,45.59% LaCl₃.There is a short metastable segment of solubility curve of LaCl₃·7H₂O (dotted line) in the diagram, which may. be considered as an extrapolated part of the stable curve of this compound.At 30rC, two phase diagrams were obtained. One was obtained just by stirring the original complexes at 30℃ and the other by preheating the original complexes up to the temperature above the m. p. of CH₃CONH₂ (81℃) and then stirring at 30℃. The former is a stable system. There are four solid phases,LaCl₃·7H₂O, LaCl₃·4CH₃CQNH₂·5H₂O, LaCl₃ and CH₃CONH₂. The eutonic solution of the first two solids contains 47.19% LaCl₃ and 14.48% CH₃CONH₂.The solubility curve of anhydrous LaCl₃ is very short and the other two eutonics can only be estimated as 33%, 59% and 31%, 66%, respectively. The second diagram (shown in dotted line) is a metastable system. Instead of the ternary compound, there is LaCl₃·3H₂O as one of the solid phases. The first two eutonic solutions now contain 47%, 29% and 43%, 46% of LaCl₃ and CH₃CONH₃, by the estimation, respectively.The density of the ternary compound determined by pycnometricalmethod, using the related mother liquor or benzene as measureing media,is about 1.70 ( 12 ± 1℃) . The photomicrograph and the X-ray diffraction patterns as well as the d/n values of the ternary compound are also given.The solid phase LaCl₃·6H₂O reported by Z. M. Zholalieva et al. in 1976 was not found both at 0℃ and at 30℃.