Abstract: The solubility of the ternary system, Y(NO₃)₃·3H₂O-B15C5-CH₃ COCH₃ at 18℃ has been determined by a phase equilibrium method. The results indicated that there are two solubility curves for the system, the long one correspounds with the solid phase of the complex,Y(NO₃)₃·B15C5·3H₂O 2.5CH₃COCH₃ and the short one with that of B15C5. There was a failure to achieve other complex species with another Y/crown ratios. The behavior of water for this system in equilibrium has been exami-ned. The results are:the mole ratio,(H₂O/Y(NO₃)₃), no matter whether it is in liquid phase or solid phase, is always 3:1. The composition of the isolated complex have been examined by chemical anaiysis, infrared spectra and thermogravimetric analysis,as well as the rela-tion between the composition of complexes and aftertreament conditions. A relatively stable complex,Y(NO₃)₃. B15C5·3H₂ O·0.65CH₃COCH₃ was obtained.