Phase transitions and morphology of poly(diethylsiloxane) have been studied with help of differential scanning calorimetry and transmission electron microscopy on carbon-platinum replicas of freeze-fractured samples. The TEM experiments showed distinct lamellae in the case of the beta and gamma crystalline polymorphs, whose thicknesses correlated well crith the calculated chain lengths, demonstrating an extended chain conformation for both polymorphs. A low heating rate dependence of the beta --> mu transition temperature in DSC and a linear relationship between isotropization temperature and reciprocal molecular weight indicated the existence of extended chain lamellae for the mesophase as well. The alpha crystalline polymorph displayed a greater softness than beta- and gamma-PDES, and no lamellar structures could be observed. DSC experiments on bimodal mixtures of PDES samples with strongly different molecular weights demonstrated molecular weight dependent phase separation in the case of beta-PDES and the occurrence of mixed lamellae for the mesophase obtained from alpha-PDES. For the latter, a less ordered packing with chain ends incorporated into the lamellae was proposed. Upon heating gamma-PDES no mesophase was formed, which was explained by surface energy effects. The critical molecular weight for mesophase formation was found to be M-n = 28 000 g/mol.