The invention provides a conjugate of a Neisseria meningitidis serogroup X capsular polysaccharide and a carrier molecule. The conjugate is typically made by (a) oxidising a primary hydroxyl group in the capsular polysaccharide, to give an oxidised polysaccharide with an aldehyde group; and (b) coupling the oxidised polysaccharide to a carrier molecule via the aldehyde group, thereby giving the conjugate. The conjugate may be part of an immunogenic composition. This composition may comprise one or more further antigens, particularly capsular polysaccharides from serogroups A, W135, C and Y and conjugated forms thereof. The composition may be in an aqueous formulation. The composition is useful as a vaccine, e.g. for raising an immune response in a mammal. The invention also provides processes for making the conjugate.