THE DISPROPORTIONATION OF MIXTURES OF BRANCHED AND UNBRANCHED DISPROPORTIONATABLE OLEFINS IN THE PRESENCE OR ABSENCE OF ETHYLENE IS IMPROVED BY ADJUSTING THE CONVERSION TO A SPECIFIED LEVEL THEREBY INCREASING THE SELECTIVELY OF THE REACTION TO THE CONVERSION OF THE UNBRANCHED OLEFINS. THE PROCESS HAS PARTICULAR UTILITY IN INCREASING OF THE QUALITY OF OLEFINIC GASOLINE STREAMS IN THAT THE LOWER OCTANE-RATED UNBRANCHED OLEFINS CAN BE SELECTIVELY REMOVED FROM THE GASOLINE BY CONVERSION TO OTHER OLEFINS WHILE SUBSTANTIALLY RETAINING THE HIGHER OCTANE-RATED BRANCHED OLEFINS. IN THIS ASPECT, A PROCESS FOR INCREASING THE OCTANCE VALUE OF OLEFINIC GASOLINE STREAMS IS PROVIDED WHICH COMPRISES THE STEPS OF SEPARATING A C5 FRACTION FROM AN OLEFINIC GASOLINE STREAM, THE C5 FRACTION CONTAINING BOTH PENTENES AND ISOPENTENES, SELECTIVELY DISPROPORTIONATING THE C5 STREAM, WITH OR WITHOUT THE PRESENCE OF ADDED ETHYLENE, TO CONVERT MORE OF THE NORMAL PENTENES THAN THE ISOPENTENES, ALKYLATING THE RESULTING BUTENES AND LIGHTER OLEFINS WITH ISOBUTANE, AND COMBINING THE ALKYLATE AND REMAINING GASOLINE TO PROVIDE A HIGH OCTANE GASOLINE HAVING A REDUCED LIGHT OLEFINS CONTENT. IN A MODIFICATION OF THE ABOVE DESCRIBED MULTISTEP PROCESS PRODUCED PROPYLENE IS DISPROPORTIONATED IN A SECOND OLEFIN DISPROPORTIONATION ZONE, TO PROVIDE ADDITIONAL BUTENES FOR ALKYLATION AND ETHYLENE OR REACTION WITH N-PENTENES IN THE FIRST STEP OF THE PROCESS.