Abstract: Relationships between the activity of a certain component and the product of ac-tivities of three components, Φ, which may be the activity of the compound, on boundaries of two-phase region in ternary systems have been studied. An important formula expressing above mentioned relationships has been derived. It is a common expression which involves various kinds of formulae for two-phase region in ternary systems. At the same time, we analyzed in detail the influence of the direction of tie-line on relationships between the activities. A parameter θ is empolyed in order to overcome the difficulties encountered in integration in the two-phase region. In addition, we consummated Krivsky and Schuhmann's rule on the basis of certain results in the paper.