A split-and-mix method for synthesising a library of different bifunctional complexes comprising a molecule and an identifier oligonucleotide, said method comprising the steps of i) dispensing in a first synthesis round a display oligonucleotide or a nascent bifunctional complex in separate compartments, ii) exposing each of said compartments to a different reactant, iii) reacting in each compartment said different reactant with at least one reactive group of a chemical reaction site on the display oligonucleotide or the nascent bifunctional complex, iv) adding in each compartment, by enzymatic or chemical ligation, an oligonucleotide tag identifying said reactant to a display oligonucleotide or a nascent bifunctional complex, wherein the substrate for the ligase is in a double stranded form, wherein the substrate comprises a plurality of tags and at least one or more anti-tags, wherein tags and anti-tag(s) is/are at least partly hybridised to each other, wherein tags are covalently linked as a result of the action of an enzyme comprising a ligase activity on the double stranded substrate, but no anti-tags are covalently linked as a result of said ligase action, v) mixing said nascent bifunctional complexes obtained in step iv), vi) dispensing said mixed nascent bifunctional complexes obtained in step v) in separate compartments, vii) repeating steps iii) and iv) for each separate compartment in a second synthesis round, viii) removing said at least one or more anti-tags, and ix) extending at least one primer and complementing a single stranded identifier oligonucleotide comprising a plurality of covalently linked oligonucleotide tags linked to the molecule. i) dispensing in a first synthesis round a display oligonucleotide or a nascent bifunctional complex in separate compartments, ii) exposing each of said compartments to a different reactant, iii) reacting in each compartment said different reactant with at least one reactive group of a chemical reaction site on the display oligonucleotide or the nascent bifunctional complex, iv) adding in each compartment, by enzymatic or chemical ligation, an oligonucleotide tag identifying said reactant to a display oligonucleotide or a nascent bifunctional complex, wherein the substrate for the ligase is in a double stranded form, wherein the substrate comprises a plurality of tags and at least one or more anti-tags, wherein tags and anti-tag(s) is/are at least partly hybridised to each other, wherein tags are covalently linked as a result of the action of an enzyme comprising a ligase activity on the double stranded substrate, but no anti-tags are covalently linked as a result of said ligase action, v) mixing said nascent bifunctional complexes obtained in step iv), vi) dispensing said mixed nascent bifunctional complexes obtained in step v) in separate compartments, vii) repeating steps iii) and iv) for each separate compartment in a second synthesis round, viii) removing said at least one or more anti-tags, and ix) extending at least one primer and complementing a single stranded identifier oligonucleotide comprising a plurality of covalently linked oligonucleotide tags linked to the molecule. <EOS>