The fire retardant action of inorganic tin additives in polyester resin thermosets cross-linked with styrene has been investigated. Thermal methods were used to analyse the degradation and oxidation index and smoke measurements were also carried out on the resins with and without tin additives. The interaction between the tin compounds and the halogens introduced into the polymer structure using halogen-containing reactive monomers affects the degradation, fire retardant and smoke suppressant properties. The changes are described and the nature of the fire retardant action is discussed.