The phase behavior of poly(4-vinylpridine) (P4VP)/poly(vinyl acetate-co-vinyl alcohol) (ACA) blends has been studied as;a function of ACA copolymer composition. Calorimetric curves for blends of ACA ranging from 23 to 53% of vinyl alcohol units show exothermal peaks above the glass transition corresponding to phase separation processes. Heats of the mixing of about 1.5 cal/g have been obtained. A progressive breaking of H-bonds as temperature increases is detected by IR spectroscopy. Cloud point curves obtained by optical microscopy measurements depend on ACA copolymer composition. The binary interaction model has been applied to this system from cloud point data.