Physicochemical and immunochemical techniques can be used to assess the quality of diphtheria toxoid vaccines. In a previous paper [Metz B, Jiskoot W, Hennink WE, Crommelin DJA, Kersten GFA. Physicochemical and immunochemical techniques predict the quality of diphtheria toxoid vaccines. Vaccine 2003;22(2):156-67], techniques were introduced which indicated toxoid quality with respect to safety and potency: SDS-PAGE, primary amino group determination, fluorescence/denaturation, circular dichroism and biosensor analyses. These analyses were performed with experimental toxoids from one toxin batch. In the present study, the quality of regular vaccine batches of different manufacturers, the properties of real-time aged products and a number of experimental toxoids were investigated, using the above-mentioned analytical techniques. We had the unique opportunity to analyse toxoids that were up to 40 years old. The real-time aged diphtheria toxoids showed hardly any structural differences as compared to the recently prepared products in both the analytical chemical techniques and the conventional potency and safety tests. The analytical assays discriminated between regular diphtheria toxoids and experimental toxoids prepared by methylation, acetylation or glutaraldehyde treatment. The analytical data showed a clear correlation with potency and safety of these toxoids. Based on the results, we refined the described physicochemical and immunochemical criteria that a standard diphtheria toxoid has to meet. We recommend further validation of these techniques for quality control of diphtheria toxoid vaccine because of their high precision and easy performance as compared to conventional in vivo procedures.