Hydroformylation process using bisphosphine monooxide ligands

In a process for producing aldehydes by the hydroformylation of an olefinically unsaturated compound with carbon monoxide and hydrogen in a reaction medium which contains a soluble rhodium-phosphorus ligand complex catalyst and free phosphorus ligand, the improvement comprising employing as the phosphorus ligand of said complex catalyst and as the free phosphorus ligand in said process an organic tertiary bisphosphine monooxide ligand having the general formula:

• ##STR24## wherein each Ar group represents an identical or different substituted or unsubstituted aryl radical, each R.sub.1 and R.sub.2 group represents an identical or different substituted or unsubstituted monovalent hydrocarbon radical and Y represents a divalent bridging group.