The present invention provides a catalyst system capable of catalyzing 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 the ligand is present in at least a 2:1 molar excess compared to the metal or the metal in the metal compound, and that the acid is present in at least a 2:1 molar excess compared to the ligand, a process for the carbonylation of an ethylenically unsaturated compound, a reaction medium, and use of the system.