A phase diagram for the commercially significant ternary system poly(acrylonitrile-co-methylacrylate-co-sulfonic acid), sodium thiocyanate, and water, used to wet-spin textile acrylic fibers, was constructed from a linearized cloudpoint curve correlation. Experimental cloudpoints in the accessible low polymer content range were evaluated and used to extrapolate to higher polymer concentrations to produce a full-phase diagram. Although the correlation only holds for systems undergoing liquid/liquid demixing, the fact that the relation holds for this system despite evidence for PAN crystallization is discussed.