Abstract: Process for the hydroformylation of ethylenically unsaturated compounds, by reaction with carbon monoxide and hydrogen in the presence of a catalyst based on a metal of the platinum group, a source of anions and a bidentate ligand of the formula R.sup.1 R.sup.2 M.sup.1 RM.sup.2 R.sup.3 R.sup.4 wherein M.sup.1 and M.sup.2 are phosphorus, arsenic or antimony, R is a bivalent organic bridging group with 1 to 4 atoms in the bridge, R.sup.1 and R.sup.2 together represent a bivalent cyclic group and R.sup.3 and R.sup.4 independently represent an optionally substituted hydrocarbyl group or together have the same meaning as R.sup.1 and R.sup.2.