Abstract The rates of production of volatile material from PVA, PVC and vinyl acetate-vinyl chloride copolymers, covering the entire composition range, bave been compared using thermal volatilization analysis. It is found that, at each extreme of the composition range, incorporation of the co-monomer unit results in a copolymer less stable than the homopolymer. Minimum stability occurs for compositions of approximately 40–50 per cent VA. The proportions of acetic acid and hydrogen chloride produced from the copolymers appear to remain constant during degradation, indicating that neither is evolved preferentially, once reaction has begun.