THERE IS DISCLOSED A PROCESS FOR THE OLYMERIZATION OF BUTADIENE AND BUTADIENE IN MIXTURE WITH OTHER CONJUGATED DIOLEFINS TO FORM POLYMERS CONTAINING A HIGH PORTION OF THE BUTADIENE UNITS IN THE CIS-1,4 CONFIGURATION COMPRISING CONTACTING AT LEAST ONE DIOLEFIN UNDER POLYMERIZATION CONDITIONS WITH A BINARY CATALYST SYSTEM COMPRISING (A) AT LEAST ONE COMPOUND SELECTED FROM A GROUP CONSISTING OF ORGANONICKEL AND ORGANOCOBALT COMPOUNDS OF FLUORINE CONTAINING (1) CARBOXYLIC ACIDS, (2) ALDEHYDES, (3) KETOMES AND (4) PHENOLS, AND (B) AT LEAST ONE ORGANOMETALLIC COMPOUND WHEREIN THE METAL IS SELECTED FROM GROUPS I, II AND III OF THE PERIODC TABLE.