Poly(vinyl chloride) (PVC) is shown to be completely miscible in the melt with a poly(methyl methacrylate-co-styrene) (P(MMA-co-St)) copolymer having a low styrene content (0-10 wt%). In blends of PVC with a low molecular weight P(MMA-co-St) copolymer containing 20 wt% of styrene, a lower critical solution temperature is found at approximately 170 degrees C. For blends of PVC with P(MMA-co-St) copolymers containing 30 wt% of styrene, immiscibility is found over the whole temperature range (150-210 degrees C) and independently of the molecular weight of the copolymer. These experimental results have been modelled using a mean-field binary interaction model of the Flory-Huggins theory. The binary interaction energy density for this blend system correctly predicts the homogeneous dispersion of methyl methacrylate/butadiene/styrene (MBS) graft copolymers in a PVC matrix.