Abstract: The invention relates to a novel olefin polymerization catalyst system comprising a reaction product of a transition metal compound. A stable and active single active site catalyst is obtained by producing said reaction product by: (I) contacting in an organic solvent the following reactive components (a) a transition metal compound, which is at least partially soluble in the organic solvent and contains in its molecule at least one organic group and a transition metal chosen from periods 4-7 and groups 3-10 of the Periodic Table (IUPAC 1990), and (b) 0.05-500 moles of an unsaturated organic compound per mole of transition metal of the transition metal compound, which unsaturated organic compound is at least partially soluble in the organic solvent, has in its molecule 2-30 carbon atoms and at least one terminal double bond,  to obtain a reaction product dissolved in the organic solvent; and (II) recovering the reaction product of the transition metal compound.