The problem of the polymer chain conformation in crystalline poly(dimethyl siloxane) has been revisited using X-ray diffraction techniques. It is shown that the crystallites belong to the centered tetragonal system. A four-fold helical configuration of the polymer chain is preposed, with four monomers per turn and a pitch of ca. 1.2 nm: it corresponds to an extended configuration of the siloxane backbone with oxygen atoms located close to the 4, symmetry axis and methyl groups pointing outwards. These results are in deep contrast with the previously accepted monoclinic structure and ribbonlike chain conformation. (C) 2000 Elsevier Science Ltd. All rights reserved.