Abstract: In hydroformylating an olefinic compound to produce an aldehyde derivative thereof employing as a catalyst a complex of a Group VIII metal and an organic ligand dissolved in an inert liquid reaction solvent, improvements in production efficiencies and product separation from reaction product in solution are achieved in using as the inert liquid reaction medium, a saturated alpha-olefin oligomer free of naphthenic and aromatic hydrocarbons having the appropriate viscosity, average molecular weights and flash point properties which are required in the hydroformylating reaction.