Abstract: In hydroformylating an olefinic compound to produce an aldehyde derivative thereof employing as catalyst a complex of a Group VIII metal and an organic ligand dissolved in an inert liquid reaction solvent with the aldehyde product being continuously stripped out of the solvent, stripping is facilitated while by-product formation is kept at a minimum by using a solvent of specially-chosen high molecular weight while also controlling the degree of stripping as desired to maintain in the reaction medium a controlled molar concentration of the aldehyde hydroformylation product.