The present invention provides a catalyst system capable of catalysing the carbonylation of an ethylenically unsaturated compound, which system is obtainable by combining: a) a metal of Group VIB or Group VIIIB or a compound thereof, b) a bidentate phosphine, arsine or stibine ligand, and c) an acid, wherein said ligand is present in at least a 2:1 molar excess compared to said metal or said metal in said metal compound, and that said acid is present in at least a 2:1 molar excess compared to said ligand, a process for the carbonylation of an ethylenically unsaturated compound, a reaction medium, and use of the system.