Through the use of a Ti(OR′)4-AlRCl2catalyst system, high 1,4-cis isoprene polymers and crystalline 1,4-trans polybutadiene are obtained. Neither monomer is polymerized at a Al/Ti mole ratio of less than 4. The maximum activity and stereospecificity for isoprene is observed at Al/Ti = 4. For 1,4-trans butadiene polymers the activity increases progressively with increasing Al/Ti ratio. The investigations carried out on this catalyst system show that at a AI/Ti mole ratio of 4 the formation of crystalline β-TiCl3takes place, while at lower ratios insoluble chloro-alkoxide derivatives of TiIIIwith different compositions separate. Soluble complexes containing aluminium and titanium are initially formed before precipitation occurs. Chemical data and investigations by IR and NMR spectroscopy indicate exchange reactions between Al-Cl, Al-R, and Ti-OR groups, together with reduction of the transition metal. A reaction mechanism and a hypothesis on the nature of the active catalyst are given.