Hydroformylation process using stable rhodium catalyst

In a process for the hydroformylation of an olefin to produce aldehydes having one more carbon atom than the olefin comprising reacting said olefin with hydrogen and carbon monoxide in a liquid reaction medium which contains a soluble rhodium complex catalyst consisting essentially of rhodium complexed with carbon monoxide and a phosphine ligand and wherein the hydroformylation reaction conditions are controlled to a temperature of from about 90.degree. to about 145.degree. C., a total gas pressure of hydrogen, carbon monoxide and olefin of less than about 450 pounds per square inch absolute, a carbon monoxide partial pressure of less than about 55 pounds per square inch absolute, a hydrogen partial pressure of less than about 200 pounds per square inch absolute, and at least about 6 total moles of said phosphine ligand for each mole of catalytically-active rhodium metal present in the rhodium complex catalyst, the improvement comprising improving the stability of said catalyst against deactivation by employing as said phosphine ligand a phosphine represented by the following formula (I):

• R.sub.n PPh.sub.3-n (I)
• wherein R represents a branched chain alkyl group having from 3 to 9 carbon atoms or a cycloalkyl group having from 5 to 12 carbon atoms, n represents an integer of 1 or 2 and Ph represents phenyl.