A process for the carbonylation of an ethylenically unsaturated compound with carbon monoxide and a co-reactant. The carbonylation reaction is carried out in the presence of a novel catalyst involving: a) a source of a group VIII metal; b) a bidentate diphosphine of formula I, wherein R1 represents a bivalent radical that together with the phosphorus atom to which it is attached is an optionally substituted 2-phospha-tricyclo[3.3.1.1{3,7}]-decyl group or a derivative thereof in which one or more of the carbon atoms are replaced by heteroatoms (“2-PA” group); wherein R2 and R3 independently represent univalent radicals of up to 20 atoms or jointly form a bivalent radical of up to 20 atoms; and wherein A1 and A2 independently represent optionally substituted alkylene groups and R represents an optionally substituted aromatic group; and, c) a source of anions.