Derivatives of PSAs are synthesised, in which a reducing and/or non-reducing end terminal sialic acid unit is transformed into a N-hydroxysuccinimide (NHS) group. The derivatives may be reacted with substrates, for instance substrates containing amine or hydrazine groups, to form non-cross-linked/crosslinked polysialylated compounds. The substrates may, for instance, be therapeutically useful drugs, peptides or proteins or drug delivery systems.