Linear polydimethylsiloxane is oxidised using atmospheric oxygen in the presence of an organic peroxide at temperatures above 180@C until the viscosity has risen to at least twice the initial value. The oxidation product is aftertreated with a low-molecular-weight silane compound which does not eliminate acidic by-products, in the presence of finely divided silicic acid at temperatures of 180@C. The viscoelastic reaction products are antifoaming agents (defoamers) of high effectiveness, in particular towards highly foaming surfactants, and have a long shelf life.
