The present invention relates to a conjugate of an active substance and hydroxyalkyl starch, wherein the active substance and the hydroxyalkyl starch are linked by a chemical moiety, having a structure according to formula (I) wherein Y is a heteroatom, selected from the group consisting of O and S, and X is selected from the group consisting of SH and said conjugate having a structure according to formula (IV) wherein HAS' is the residue of the hydroxyalkyl starch or derivative thereof which was linked to the thioester group, and wherein AS' is the residue of the active substance or a derivative thereof which was linked to the alpha-X beta-amino group, or a structure according to formula (V) wherein HAS' is the residue of the hydroxyalkyl starch or derivative thereof which was linked to the alpha-X beta-amino group, and wherein AS' is the residue of the active substance or a derivative thereof which was linked to the thioester group, and wherein the group -(C=Y) is derived from the thioester group -(C=Y)-S-R' and the group HN-CH-CH 2 -X is derived from the alpha-X beta amino group.
