A predictive scheme is proposed for the simultaneous calculation of the modulus and yield (or tensile) strength of ternary polymer systems. According to the continuity or discontinuity of constituting phases, the scheme combines in two steps the models for binary systems: (i) in the interval of phase duality (co-continuity), a twoparameter equivalent box model is used along with the data on the phase continuity rendered by modified equations of the percolation theory; and (ii) the effects of a dispersed phase on the mechanical properties of a continuous phase are treated by using the approach developed earlier for particulate systems. Simultaneously predicted values of the modulus and yield (or tensile) strength of ternary systems are interrelated because they are calculated by using an identical set of input parameters characterizing a specific phase structure. The predictive scheme will allow the experimentalists: (i) to anticipate selected mechanical properties of envisaged blends (for presumed phase structures); (ii) by comparing experimental and theoretical data, to assess to which percentage the potential of a material has been exploited; (iii) to analyze the phase structure of prepared ternary blends; and (iv) to evaluate interfacial adhesion or the extent of interfacial debonding. The versatility of the predictive scheme is demonstrated on three examples of various types of ternary systems.