An approach was developed to align release and end-expiry specifications for molecular size for the polyvalent pneumococcal polysaccharide vaccine (PNEUMOVAX ® 23). Each of the 23 polysaccharide components of the vaccine was separately subjected to ultrasonication to produce a series of preparations of decreasing weight-average molecular mass (M w ). These size-reduced polysaccharides were analysed as monovalent solutions by high-performance size exclusion chromatography (HPSEC) with multi-angle laser light scattering (MALLS) and refractive index (RI) detection to measure their M w . These samples were also analysed by HPSEC with rate nephelometry (RN) detection to measure their relative molecular size (r-MS). The data from the two molecular size measurements established a correlation between M w and r-MS. For each polysaccharide component of the vaccine, this correlation permits the direct alignment of the r-MS specification in the final formulated product with the M w specification for the monovalent polysaccharide preparation. The alignment of specifications provides a high level of assurance that the quality control of the final vaccine product is consistent with that of the polysaccharide starting materials.