Abstract Fire retardance mechanism in polypropylene/chloroparaffin/metal chloride systems (PP/CP/MC) using different chloride or chloride suppliers has been investigated and a synergistic effect has been recognised in some cases, which mainly involves a condensed phase effect. By studying PP/MC and CP/metal (0) mixtures separately it has been found that some MC are reduced by PP to M (0) (Cu +, Cu ++, Sn 4+, Sb 3+, Bi 3+) and some M (0) are oxidised by CP (Sn, Sb, Bi) depending on their standard reduction potential. The reaction between PP and MC, which consumes PP radicals reducing the rate of thermal oxidation of PP, is responsible for the condensed phase effect of fire retardance. When both reduction of MC by PP and oxidation of M by CP is allowable (Sn, Sb, Bi) a catalytic mechanism is established which leads to the synergistic effect of fire retardance by regenerating the MC reduced by PP. Synergistic effect with Cu compounds is explainable by involvement of Cu +/Cu ++ redox couple; Cd and Pb compounds does not show any synergistic effect.