New formulae for calculating activities of components from a binary simple eutectic phase diagram are presented in this paper. In the new formulae, activity coeffi cients are considered as functions of the composition of the system. Application of these formulae to system Cu-Bi shows that the new formulae are satisfatory. New formulae for calculating activities of components from a binary simple eutectic phase diagram are presented in this paper. In the new formulae, activity coeffi cients are considered as functions of the composition of the system. Application of these formulae to system Cu-Bi shows that the new formulae are satisfatory.