The present invention provides a process for upgrading a heavy hydrocarbon mixture, said process comprising: i) dividing said heavy hydrocarbon mixture into at least a first portion and a second portion, wherein said first portion comprises 10-45% wt of the heavy hydrocarbon mixture and said second portion comprises 90-55% wt of the heavy hydrocarbon mixture; ii) thermally upgrading said first portion of heavy hydrocarbon mixture in an upgrader to produce a lighter hydrocarbon mixture; and iii) mixing said lighter hydrocarbon mixture with a heavy hydrocarbon mixture to produce an upgraded hydrocarbon mixture, wherein, on entry to said upgrader, the composition of said first portion of heavy hydrocarbon mixture is identical to that of said second portion of heavy hydrocarbon mixture.