Conjugates of an active substance and hydroxyalkyl starch (HAS) are provided herein. The active substance and the HAS are linked by a chemical moiety having a structure according to formula (I) wherein Y is O or S, and X is SH or (F). The conjugate has a structure according to formula (IV) wherein HAS′ is a residue of HAS or a derivative thereof linked to the thioester group, and AS′ is a residue of the active substance or derivative thereof linked to the alpha-X beta-amino group or a structure according to formula (V) wherein HAS′ is a residue of HAS or derivative thereof linked to the alpha-X beta-amino group, and AS′ is a residue of the active substance or derivative thereof 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—CH2—X is derived from the alpha-X beta amino group.
