The present invention relates to a method of production of a hydrazide modified sugar comprising a step of reacting a sugar with a hydrazide in a reaction solvent at a pH of between 3 and 5.5, wherein the solvent comprises an aqueous based solvent and an optional polar organic co-solvent. A further aspect of the invention relates to a method of production of a polysaccharide epitope carrier protein conjugate comprising the steps of: (a) reacting a polysaccharide epitope with a hydrazide to form a hydrazide modified polysaccharide epitope; (b) reacting the hydrazide modified polysaccharide epitope with a linker that has been pre-coupled to a carrier protein. Another aspect of the invention relates to a method of production of a sugar-dihydrazide-aldehyde adduct comprising the steps of: (a) producing a hydrazide modified sugar using a method according to the invention, wherein the hydrazide modified sugar includes a further unreacted hydrazide moiety; and (b) reacting the further hydrazide moiety with the aldehyde functionality of a linker group.