Abstract An inert metal oxide (TiO 2), a catalytic oxide (Fe 2O 3) and an oxide which forms volatile halides (MoO 3) have been incorporated into polypropylene and polystyrene on their own, in combination with one another, and in the absence and presence of a halogen compound (Cereclor 70 or decabromobiphenyl oxide). Studies of the resulting flame-retardant and smoke-suppressant effects of the additive systems have been carried out by means of triangular diagrams. None of the systems investigated has any sizeable effect on the flammability of polypropylene, the limiting oxygen indices being raised by no more than 8 units. A positive interaction between Cereclor 70 and molybdenum oxide results however both in an improvement in the degree of flame retardance of this polymer and in a decrease in its already rather low smoke-producing tendency. These additive systems have proved more effective in the case of polystyrene. The decreased flammability manifests itself as an increase of up to 12 LOI units, while the maximum smoke density resulting from the combustion of the polymer is considerably decreased. In some cases the maximum smoke density, D s, is less than 5% of the value for the pure polymer. The most efficient flame-retardant and smoke-suppressant systems generally involve molybdenum oxide and these are, in particular, ternary systems containing also another metal oxide and decabromobiphenyl oxide. Iron(III) oxide has little effect on its own but nevertheless considerably enhances the activity of the other metal oxides.