The invention provides a process for the reductive amination of a carbonyl group at the reducing terminus of a polysaccharide, wherein the reductive amination is carried out at a pH between 4 and 5. The invention also provides a process for preparing a conjugate of a polysaccharide and a carrier molecule, comprising the steps of: (a) coupling the polysaccharide to a linker, to form a polysaccharide-linker compound in which the free terminus of the linker is an ester group； and (b) reacting the ester group with a primary amine group in the carrier molecule, to form a polysaccharide-linker-carrier molecule conjugate in which the linker is coupled to the carrier molecule via an amide linkage. The invention also provides a process for reducing contamination of a polysaccharide-linker compound with unreacted linker, comprising a step of precipitating unreacted linker under aqueous conditions at a pH of less than 5. The invention also provides polysaccharide-linker-carrier molecule conjugates and intermediate compounds obtained or obtainable by these processes.
