Method for the electrolytic surface modification of a flat metal workpiece, in which at least one surface of the flat metal workpiece is anodically poled in a treatment liquid and an anodic dissolving process is thereby induced, and then the at least one surface of the flat metal workpiece is cathodically poled in a treatment liquid and a cathodic deposition process is thereby induced for the deposition of one or more metals on the at least one surface of the flat metal workpiece, wherein the treatment liquid in the anodic dissolving process and the treatment liquid in the cathodic deposition process are different treatment liquids and the treatment liquid in the anodic dissolving process and the treatment liquid in the cathodic deposition process are separated by an electrically non-conductive separating liquid which has contact to the flat workpiece, and wherein the treatment liquid in the anodic dissolving process and/or the treatment liquid in the cathodic deposition process is an electrically conductive liquid comprising an aqueous solution of a metal complex, wherein the metal complex is a complex of (i) one or more metals selected from the group consisting of Cu, Zn, Mn, In, Sb, Bi, Fe, Ni, Co, Ti, Zr, Nb, Y, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or mixtures thereof, and (ii) one or more diphosphonate ligands of the general formula O=P 2 -X- 2 P=O wherein the OH groups in the general formula which are bonded to the two phosphorus atoms independently of each other are protonated or deprotonated (O -), wherein: X = O, NR 1 or CR 1 R 2, in particular X = CR 1 R 2, R 1 = H, C 1 -C 18 -n-alkyl or C 3 -C 18 -isoalkyl, C 5 -C 6 -cycloalkyl, unsubstituted or substituted benzyl and substituted or unsubstituted phenyl, R 2 = R 1, -OR 3 or -NHR 3, and R 3 = H, C 1 -C 4 -n-alkyl or C 3 -C 4 -isoalkyl, and wherein the OH groups in the general formula which are bonded to the two phosphorus atoms independently of each other are protonated or deprotonated (O -), wherein the liquid furthermore comprises an additive of the general formula: R 10 -CHR 8 -CHR 9 -Z-(CHR 4 -CHR 5 -Z) n -CHR 6 -CHR 7 -R 10 wherein: n = an integer from 1 to 11, in particular an integer from 1 to 3, Z = S or O, in particular S, R 4 = H, C 1-4 -alkyl or phenyl, R 5 = H, C 1-4 -alkyl or phenyl, R 6 = H, C 1-4 -alkyl or phenyl, R 7 = H, C 1-4 -alkyl or phenyl, R 8 = H, C 1-4 -alkyl or phenyl, R 9 = H, C 1-4 -alkyl or phenyl, R 10 = OH, COOH or COOR 11 and R 11 = alkyl, Li, Na, or K, wherein the additive is present in an amount of from 0.05 to 0.7 wt.%, based on the weight of the total solution. (i) one or more metals selected from the group consisting of Cu, Zn, Mn, In, Sb, Bi, Fe, Ni, Co, Ti, Zr, Nb, Y, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or mixtures thereof, and (ii) one or more diphosphonate ligands of the general formula O=P 2 -X- 2 P=O wherein the OH groups in the general formula which are bonded to the two phosphorus atoms independently of each other are protonated or deprotonated (O -), wherein: X = O, NR 1 or CR 1 R 2, in particular X = CR 1 R 2, R 1 = H, C 1 -C 18 -n-alkyl or C 3 -C 18 -isoalkyl, C 5 -C 6 -cycloalkyl, unsubstituted or substituted benzyl and substituted or unsubstituted phenyl, R 2 = R 1, -OR 3 or -NHR 3, and R 3 = H, C 1 -C 4 -n-alkyl or C 3 -C 4 -isoalkyl, and wherein the OH groups in the general formula which are bonded to the two phosphorus atoms independently of each other are protonated or deprotonated (O -), X = O, NR 1 or CR 1 R 2, in particular X = CR 1 R 2, R 1 = H, C 1 -C 18 -n-alkyl or C 3 -C 18 -isoalkyl, C 5 -C 6 -cycloalkyl, unsubstituted or substituted benzyl and substituted or unsubstituted phenyl, R 2 = R 1, -OR 3 or -NHR 3, and R 3 = H, C 1 -C 4 -n-alkyl or C 3 -C 4 -isoalkyl, and wherein the OH groups in the general formula which are bonded to the two phosphorus atoms independently of each other are protonated or deprotonated (O -), n = an integer from 1 to 11, in particular an integer from 1 to 3, Z = S or O, in particular S, R 4 = H, C 1-4 -alkyl or phenyl, R 5 = H, C 1-4 -alkyl or phenyl, R 6 = H, C 1-4 -alkyl or phenyl, R 7 = H, C 1-4 -alkyl or phenyl, R 8 = H, C 1-4 -alkyl or phenyl, R 9 = H, C 1-4 -alkyl or phenyl, R 10 = OH, COOH or COOR 11 and R 11 = alkyl, Li, Na, or K,