The chemical activity of mineral fillers, under conditions similar to those encountered in filled-polymer systems, has been studied. It is shown that, contrary to previous views, the alumino-silicates have strongly acidic surfaces and that this surface acidity influences the chemical reactions in polymeric composites. Although the number of acid sites is significantly less than those of cracking catalysts, the strength of the sites is of a similar order of magnitude. A study of the infrared spectra of pyridine adsorbed on kaolin has shown that both Lewis and Brønsted acid sites are present. The ratio of Lewis to Brønsted acid sites varies with the free moisture content of the clay.