A study has been made of the nature of active sites, stereospecificity of their action and the regularities of diene polymerization catalysed by chromium.containing systems. All possible polymer structures with high stereospecificity can be produced for butadiene and isoprene with n-allyl chromium compounds. Tris-n-allyl chromium produces polybutadiene predominantly of 1,2-units. Cis-polybutadiene is formed when the electronegative group (CI-, CCI3COO-) is substituted for one or two n-allyl groups in Tris-allyl chromium or in the catalytic system (n-C3HshCr-AI203. A catalyst obtained through interaction of (n-C3Hs)3Cr with silica-alumina or silica gel produces 1,4-trans-polybutadiene and 1,4trans-polyisoprene. The rate of butadiene polymerization in the presence of Tris-n-allyl chromium is given by k[Cr] 2, and in polymerization of isoprene with the catalytic system (n-C3Hs)3Cr-silica-alumina, by k[Cr].[M] 2. Polymerization of dienes catalysed by (n-C3Hs)3Cr-silica-alumina system or supported chromium oxide catalyst proceeds according to a type of living system. A study was made of copolymerization of butadiene and isoprene in the presence of supported chromium oxide catalyst and with that produced by the reaction of (n-C3H 5)aCr with silica-alumina. The constants of eopolymerization for the systems were equal. A conclusion has been drawn regarding the similar mechanisms for diene polymerization under the action of supported chromium oxide catalyst or of catalyst formed in the reaction of (n-C3H~)3Cr with silica-alumina or silica gel.