A conjugate comprised of a hydrophilic polymer covalently yet reversibly linked to a amine-, hydroxy- or carboxyl-containing ligand is described. The resulting conjugate is capable of releasing the parent amine, hydroxy, or carboxyl-containing compound via thiol-mediated cleavage. The system allows for delivery of various amino-, hydroxy-, or carboxy-containing drugs in the form of their thiolytically cleavable macromolecular conjugates.