Process comprising the process steps of: a) initially charging an ether having from 3 to 30 carbon atoms; b) adding a phosphine ligand and a compound comprising Pd, or adding a comprising Pd and a phosphine ligand; b') adding an acid to the reaction mixture; c) feeding in CO; d) heating the reaction mixture, with conversion of the ether; wherein the phosphine ligand is a compound of formula where m and n are each independently 0 or 1; 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 is a-(C 3 -C 20)-heteroaryl radical; and 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 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, -SO 3 H, -NH 2, halogen; and wherein no alcohol is added to the reaction mixture. a) initially charging an ether having from 3 to 30 carbon atoms; b) adding a phosphine ligand and a compound comprising Pd, or adding a comprising Pd and a phosphine ligand; b') adding an acid to the reaction mixture; c) feeding in CO; d) heating the reaction mixture, with conversion of the ether; wherein the phosphine ligand is a compound of formula where m and n are each independently 0 or 1; 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 is a-(C 3 -C 20)-heteroaryl radical; and 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 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, -SO 3 H, -NH 2, halogen; and wherein no alcohol is added to the reaction mixture. m and n are each independently 0 or 1; 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 is a-(C 3 -C 20)-heteroaryl radical; and 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 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, -SO 3 H, -NH 2, halogen; and wherein no alcohol is added to the reaction mixture.