The invention provides a process for preparing a conjugate of a S. aureus type 5 or type 8 capsular polysaccharide and a carrier molecule, comprising the steps of: (a) depolymerizing the capsular polysaccharide, to give a polysaccharide fragment; (b) oxidizing the fragment in order to introduce an aldehyde group into at least one saccharide residue in the fragment, to give an oxidized saccharide residue; and (c) coupling the oxidized saccharide residue to a carrier molecule via the aldehyde group, thereby giving the conjugate. The coupling in step (c) may be direct, or may be via a linker molecule. The invention also provides a conjugate obtained or obtainable by this process.