The invention relates to a process for hydroformylating olefins with carbon monoxide and a hydrogen atom source in the presence of a catalyst system obtainable by combining (a) a source of a Group VIII metal cation, (b) a compound acting as bidentate ligand of the formula R.sup.1 R.sup.2 M.sup.1 --R--M.sup.2 R.sup.3 R.sup.4, wherein M.sup.1 and M.sup.2 independently are P, As or Sb, R represents a bivalent substituted or non-substituted bridging group containing from 1 to 5 atoms in the bridge, R.sup.1 and R.sup.2 together are a substituted or non-substituted bivalent group whereby the two free valencies are linked to M.sup.1, and R.sup.3 and R.sup.4 together are a substituted or non-substituted bivalent group whereby the two free valencies are linked to M.sup.2 or R.sup.3 and R.sup.4 independently are substituted or non-substituted hydrocarbyl groups, and (c) a source of anions; wherein the hydrogen atom source comprises water or a primary or secondary alcohol.