A process for oligomerizing the olefins present in a Fischer-Tropsch derived condensate containing a mixture of olefins and oxygenates and boiling in the range of diesel which comprises (a) removing substantially all of the oxygenates present in the Fischer-Tropsch condensate; (b) contacting the Fischer-Tropsch derived condensate having less than 100 ppmw elemental oxygen with an ionic liquid catalyst in an oligomerization zone under oligomerization reaction conditions; and (c) recovering from the oligomerization zone a Fischer-Tropsch derived product having molecules characterized by a higher average molecular weight and increased branching as compared to the Fischer-Tropsch derived condensate.