AbstractStudies have been made of the influence on smoke generation from polystyrene of the incorporation of a number of apparently inert solids, the smoke‐producing tendency being expressed in terms of the maximum specific optical density of smoke per unit weight of polymer consumed. Of the compounds investigated, by far the most efficient smoke suppressant is pyrogenic silica, the effect of which is, over a wide range, directly proportional to the total surface area of the material introduced. Examination of partially burned polystyrene‐pyrogenic silica composites reveals the presence at the surface of a hard rigid skin, which is not observer with polystyrene alone of with polymer samples containing other additives. The formation of a protective skin is also indicated by the kinetics of forced burning of these composites where the weight loss is a linear function of log time and by measurements of the rate of change of thickness of polymer samples during combustion. Thermogravimetric experiments in which polystyrene and pyrogenic silica are heated in contact with one another show that the silica encourages the formation of a carbon residue from the polymer. A mechanism of skin production is proposed which involves the build‐up of layers of tangential spherical silica particles cemented together by cross‐linked poly