Derivation of polynomial equations as a function of two concentration variables to determine the activities of the components of a multicomponent system from the knowledge of its gas solubility. Development of polynomial functions to describe the hydrogen solubility of the mixed phase. Application of the polynomial equations to calculate the activity coefficients of the components iron, cobalt and silicon of the liquid ternary system iron–cobalt–silicon. Representation and discussion of the activities as isothermal planes as a function of two concentration variables.