IN THE POLYMERIZATION OF AN OLEFIN, E.G., PROPYLENE, EMPLOYING HYDROGEN AS A MODIFIER IN THE POLYMERIZATION REACTION AND EXCESS MONOMER AS SOLVENT TO PRODUCE A DESIRED MELT INDEX POLYMER, RECYCLED UNPOLYMERIZED OR EXCESS MONOMER IS COMPLETELY FREED OF HYDROGEN BY FRACTIONATION OF THE RECOVERED MONOMER STREAM BY SUBMITTING THE SAME TO A DEGASSING OPERATION IN WHICH IN THE PREFERRED MODE OF OPERATION THE STREAM IS HEATED IN A FRACTIONATION ZONE TO EXPEL HYDROGEN THEREFROM AND THE OVERHEAD IS REFRIGERATED TO AVOID UNDUE LOSS OF MONOMER AND ALL OF THE MODIFIER HYDROGEN IS ADDED TO THE POLYMERIZATION REACTION FROM A SINGLE SOURCE.