An algorithm is considered for the calculation of solubility isotherm-isobars for ternary systems with various types of solid solutions: without discontinuities; or with a discontinuity resulting from diffusion instability, different crystal-structure types, or different hydrate compositions of the components. Solubility curves are calculated for ternary systems KCl-KBr-H₂O, NH₄Cl-NH₄Br-H ₂O, KCl-NH₄Cl-H₂O, and KBr-NH ₄Br-H₂O at 25°C.