Abstract: A process for the hydroformylation of an olefin, which comprises hydroformylating an olefinic compound by reacting it with carbon monoxide and hydrogen in a hydroformylation reaction zone in the presence of a catalyst solution containing rhodium and an oxide of an organic trivalent phosphorus compound to form an aldehyde having a carbon number larger by one than that of the olefinic compound; adding an organic trivalent phosphorus compound to the reaction mixture, followed by distillation to separate a distillate containing the aldehyde from a high-boiling residue containing rhodium; oxidizing the high-boiling residue at the same time as or subsequent to the distillation to convert the organic trivalent phosphorus compound to its oxide; and recycling the oxidized residue to the hydroformylation reaction zone as a recycling catalyst solution, wherein the hydroformylation reaction is conducted under a condition such that the proportion of an alcohol content supplied as contained in the recycling catalyst soluti