Abstract: A process is provided, in particular a continuous process for the carbonylation of butadiene by reacting the butadiene with carbon monoxide and a hydroxyl group-containing compound in the presence of a palladium catalyst system in a reaction zone to form a reaction mixture, said catalyst system comprising (a) a source of palladium cations, (b) a mono-, bi- or multidentate phosphine ligand, containing at least one phosphorus atom which is directly bound to two or three aliphatic carbon atoms, as process ligand to produce a palladium-phosphine ligand complex catalyst, and (c) a source of anions, said process ligand (b) containing the moiety shown in formula (I), wherein A1 and A2 each represent an aliphatic carbon atom which can be connected to one or more aliphatic or aromatic carbon atoms or both A1 and A2 are part of an at least 5-membered ring including the phosphorus atom, and X represents an aliphatic or aromatic carbon atom which can be connected to one or more aliphatic or aromatic carbon atoms or