The thermal stability of poly(vinyl chloride) (PVC)/poly(alpha-methylstyrene-co-acrylonitrile) blends was studied by thermogravimetry. The stability of the blends is different from that predicted by the additivity rule. In general, the blends are less stable at lower temperatures but more stable at higher temperatures than expected. The stability of PVC-rich blends is also affected by their miscibility behavior.