Process for preparing a synthesis gas suitable as starting material for the oxo-synthesis

In an oxo process wherein an olefin is hydroformylated to an oxygen-containing product having one or more carbon atom than said olefin, and wherein after recovery of said product there remains a thick oil residue comprising aldehydes, alcohols, acetals, aldols and esters, the improved method of converting said thick oil to useful product comprising cracking said thick oil over a supported nickel catalyst containing in the range of 2 to 25 weight percent nickel in the presence of in the range of 1.5 to 2.5 moles water vapor and 1.6 to 3.0 moles carbon dioxide per g-atom in said thick oil, and at a temperature in the range of 600.degree. C to 900.degree. C and a pressure in the range up to 30 atmospheres, recovering from the material so cracked a synthesis gas mixture consisting essentially of carbon monoxide and hydrogen, and controlling the amount and properties of said water vapor and carbon dioxide used to yield a hydrogen/carbon monoxide ratio in said synthesis gas mixture in the approximate range of 1:1 to 2:1 and rcycling said hydrogen and carbon monoxide to said oxo process.