AbstractThe effects of particle size and concentration of calcium carbonate fillers on the critical oxygen index and the evolution of hydrogen chloride during pyrolysis of PVC have been examined. Both retention of hydrogen chloride in the condensed phase and critical oxygen index chloride in the condensed phase and critical oxygen index depend not only on concentration but also on particle size of the calcium carbonate. Efficiency of retention of hydrogen chloride increases with decreasing particle size and approaches 100% at particle sizes below 0.1 micron. Critical oxygen index is not appreciably affected by calcium carbonates of particle size around one micron. Coarser calcium carbonates increase critical oxygen index, while finer calcium carbonates decrease it up to concentrations at which all the hydrogen chloride is retained in the condensed phase, after which it is steadily increased with increasing concentration.