We have classified a number of aqueous ternary systems containing two different polymers into three types by focusing on the deviation of the Huggins constant k´ from the additivity line. Systems of type I have negative deviations of k´; the repulsive interaction between the two different polymers dominates. In systems of type II, k´ almost follows the additivity relation; the repulsive and attractive interactions between the two different polymers are balancing. Type III systems have positive deviations of k´; the attractive interactions are relatively dominant. This classification of systems is supported by the fact that the positive and negative deviations of k´ from the additivity line also correspond to the sign of interaction parameter between polymer 2 and 3, .Δb₂₃. Furthermore, we have verified the relatively high compatibility between dextran and poly(vinyl alcohol) by determining the binodal concentration of a liquid-liquid phase separation for a water/dextran/poly(vinyl alcohol) system, which is classified as type III. Thus, we found that the compositional dependence of k´ closely relates to the compatibility of binary polymer mixtures in aqueous ternary systems.