A process is disclosed for the carbonylation of acetylenically unsaturated compounds by reaction with carbon monoxide and a co-reactant in the presence of a catalyst system based on: (a) a source of platinum; (b) a 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.