AbstractThe Flory‐Huggins interaction parameter,x, has been determined as a function of plasticizer composition, ϕ2, for several ternary mixtures of poly(vinyl chloride) (PVC) and different pairs of plasticizers, selected mainly from the family of aliphatic esters. The method employed was the micro‐determination of the apparent melting temperature of a PVC particle in excess plasticizer. In some systemsXchanged fairly gradually as a function of ϕ2, while other systems exhibited a pronounced minimum in theX– ϕ2plots. Attention has been drawn to the similarity of this latter behavior to the well known co‐solvency effect. The existence of such minima is of partical interest in plasticized PVC fo