Abstract: A process for the hydroformylation of ethylenically unsaturated compounds with carbon monoxide and hydrogen in the presence of a catalyst system comprising:a) a source of palladium, platinum, or nickel cations;b) a source of anions, other than halide anions, such as unsubstituted alkyl sulfonic acids, aryl sulfonic acids, perfluorinated alkyl sulfonic acids, perfluorinated aryl sulfonic acids, boric acid derivatives or alkylated versions thereof;c) a source of at least one bidentate ligands of the formulaR.sup.1 R.sup.2 M.sup.1 RM.sup.2 R.sup.3 R.sup.4 (I)wherein M.sup.1 and M.sup.2 independently represent a phosphorus, arsenic or antimony atom, R represents a bivalent bridging group containing from 1-4 atoms in the bridge, R.sup.1 and R.sup.2 together represent a bivalent cyclic group whereby the two.cndot.free valencies are linked to M.sup.1, and R.sup.3 and R.sup.4 independently represent a hydrocarbyl group, or together represent a bivalent cyclic group whereby the two free valencies are linked to M.sup.