Process comprising the following process steps: a) introducing an ethylenically unsaturated compound; b) adding a ligand-metal complex comprising Pd and a bidentate phosphine ligand, or adding a bidentate phosphine ligand and a compound which comprises Pd; c) adding an alcohol; d) supplying CO; e) heating the reaction mixture, the ethylenically unsaturated compound being reacted to form an ester, where the reaction mixture is admixed with less than 0.1 mol%, based on the amount of substance of the ethylenically unsaturated compound, of Brønsted acids having an acid strength of pKa ≤ 3, characterized in that the phosphine ligand is substituted on at least one phosphorus atom by at least one heteroaryl radical; wherein the bidentate phosphine ligand is selected from a compound according to one of the formulae and where R 1, R 2, R 3, R 4, R 1', R 2', R 3', R 4' are each independently selected from -(C 1 -C 12)-alkyl, -(C 3 -C 12)-cycloalkyl, -(C 3 -C 12)-heterocycloalkyl, -(C 6 -C 20)-aryl, -(C 3 -C 20)-heteroaryl; at least one of the R 1, R 2, R 3, R 4 radicals or at least one of the R 1', R 2', R 3', R 4' radicals is a -(C 3 -C 20)-heteroaryl radical; and R 1, R 2, R 3, R 4, R 1', R 2', R 3', R 4', if they are -(C 1 -C 12) -alkyl, -(C 3 -C 12)-cycloalkyl, -(C 3 -C 12)-heterocycloalkyl, -(C 6 -C 20)-aryl or -(C 3 -C 20)-heteroaryl, may each independently be substituted by one or more substituents selected from -(C 1 -C 12)-alkyl, -(C 3 -C 12)-cycloalkyl, -(C 3 -C 12)-heterocycloalkyl, -O-(C 1 -C 12)-alkyl, -O-(C 1 -C 12)-alkyl- (C 6 -C 20)-aryl, -O-(C 3 -C 12)-cycloalkyl, -S-(C 1 -C 12) -alkyl, -S-(C 3 -C 12)-cycloalkyl, -COO-(C 1 -C 12)-alkyl, -COO-(C 3 -C 12)-cycloalkyl, -CONH- (C 1 -C 12)-alkyl, -CONH-(C 3 -C 12)-cycloalkyl, -CO-(C 1 -C 12)-alkyl, -CO-(C 3 -C 12) -cycloalkyl, -N-[(C 1 -C 12)-alkyl] 2, -(C 6 -C 20)-aryl, -(C 6 -C 2)-aryl- (C 1 -C 12)-alkyl, -(C 6 -C 20)-aryl-O-(C 1 -C 12)-alkyl, -(C 3 -C 20)-heteroaryl, -(C 3 -C 20)-heteroaryl- (C 1 -C 12)-alkyl, -(C 3 -C 20)-heteroaryl-O-(C 1 -C 12)-alkyl, -COOH, -OH, -SO 3 H, -NH 2, halogen. a) introducing an ethylenically unsaturated compound; b) adding a ligand-metal complex comprising Pd and a bidentate phosphine ligand, or adding a bidentate phosphine ligand and a compound which comprises Pd; c) adding an alcohol; d) supplying CO; e) heating the reaction mixture, the ethylenically unsaturated compound being reacted to form an ester, R 1, R 2, R 3, R 4, R 1', R 2', R 3', R 4' are each independently selected from -(C 1 -C 12)-alkyl, -(C 3 -C 12)-cycloalkyl, -(C 3 -C 12)-heterocycloalkyl, -(C 6 -C 20)-aryl, -(C 3 -C 20)-heteroaryl; at least one of the R 1, R 2, R 3, R 4 radicals or at least one of the R 1', R 2', R 3', R 4' radicals is a -(C 3 -C 20)-heteroaryl radical; and R 1, R 2, R 3, R 4, R 1', R 2', R 3', R 4', if they are -(C 1 -C 12) -alkyl, -(C 3 -C 12)-cycloalkyl, -(C 3 -C 12)-heterocycloalkyl, -(C 6 -C 20)-aryl or -(C 3 -C 20)-heteroaryl, may each independently be substituted by one or more substituents selected from -(C 1 -C 12)-alkyl, -(C 3 -C 12)-cycloalkyl, -(C 3 -C 12)-heterocycloalkyl, -O-(C 1 -C 12)-alkyl, -O-(C 1 -C 12)-alkyl- (C 6 -C 20)-aryl, -O-(C 3 -C 12)-cycloalkyl, -S-(C 1 -C 12) -alkyl, -S-(C 3 -C 12)-cycloalkyl, -COO-(C 1 -C 12)-alkyl, -COO-(C 3 -C 12)-cycloalkyl, -CONH- (C 1 -C 12)-alkyl, -CONH-(C 3 -C 12)-cycloalkyl, -CO-(C 1 -C 12)-alkyl, -CO-(C 3 -C 12) -cycloalkyl, -N-[(C 1 -C 12)-alkyl] 2, -(C 6 -C 20)-aryl, -(C 6 -C 2)-aryl- (C 1 -C 12)-alkyl, -(C 6 -C 20)-aryl-O-(C 1 -C 12)-alkyl, -(C 3 -C 20)-heteroaryl, -(C 3 -C 20)-heteroaryl- (C 1 -C 12)-alkyl, -(C 3 -C 20)-heteroaryl-O-(C 1 -C 12)-alkyl, -COOH, -OH, -SO 3 H, -NH 2, halogen.