AbstractIn view of the current demand for novel, non‐toxic, flame‐ and smoke‐ suppressant systems for synthetic polymers, certain inorganic tin compounds have been evaluated as fire retardants in a series of commercial brominated polyester resin formulations. The results obtained clearly show that zinc hydroxystannate (ZnSn(OH)6) and zinc stannate (ZnSnO3) impart beneficial properties to the polyesters in terms of flame retardancy and smoke/carbon monoxide suppression, and the improvements in performance are, in general, superior to those exhibited by tin (IV) oxide or antimony (III) oxide. The surface area and degree of dispersion of the fire‐retardant additive has been shown to have a marked effect on its efficiency and, in this connection, colloidal tin (IV) oxide is found to exhibit significantly improved flame‐retardant properties compared to powdered SnO2. Simultaneous thermal analyses (TG/DTG/DTA) and related mechanistic experiments have shown that tin additives markedly increase the amount of char formed during combustion, whereas Sb2O3, a vapour‐phase flame retardant, has little effect on char formation. The zinc stannates also appear to exhibit a significant vapour‐phase activity, and this may account for their flame‐retardant superiorit