The present invention relates to a process for the production of a conjugate from a polynucleotide and a polysaccharide comprising the steps: a) provision of an aldonic acid of the polysaccharide or of a derivative thereof; b) reaction of the aldonic acid with an alcohol derivative, preferably a carbonate derivative of an alcohol, to an aldonic acid ester, preferably to an activated aldonic acid ester; and c) reaction of the aldonic acid ester with the polynucleotide, wherein the polynucleotide exhibits a functional amino group, wherein the reaction of the aldonic acid with the alcohol derivative in step b) takes place in a dry aprotic polar solvent.