Abstract: By using the heptahydrate and trihydrate of lathanum trichloride as starting materials, the solubilities and the refractive indices of the saturated solutions in the ternary system LaCl₃-CS(NH₂)₂-H₂O have been investigated. The isotherms of solubilities and refractive indices of the system at both 30℃ and 50℃ consist of 2 branches, one corresponding to the saturated solution of LaCl₃·7H₂O and the other to CS(NH₃)₂. The solubility data obtained are: LaCl₃ - 49.32% and CS(NH₂)₃ -16.83% at 30℃ and 51.72% and 32.46% at 50° in pure water, respectively. The compositions of the double saturated solutions are: LaCl₃ - 46.72%, CS(NH₂)₂-6.15% at 30℃ and 48.05% and 8.98% at 50℃, respectively.The presence of solid phase, hexahydrated LaCl₃ reported by Z. M. Zholalieva et al, in 1978 was not found.