The present invention provides an improved method for the purification of a mixture of complexes comprising a stress protein complexed to a peptide or peptide fragment from a source mixture, typically a cell lysate. The method of the invention provides for protein complexes to be purified using ion exchange chromatography based methods, wherein a modified buffer solution is used which results in the purified stress protein complexes being more immunogenic than protein complexes obtained using conventional methodology. The purified complexes can be used to produce improved vaccine preparations which elicit enhanced immune responses in the subjects to whom the vaccine compositions are administered.