Optimizing the preservative regime for a preparation requires the antimicrobial effectiveness of several preservative combinations to be determined. In this study, three preservatives were tested: benzoic acid, sorbic acid and benzylic alcohol. Their preservative effects were evaluated using the antimicrobial preservative efficacy test (challenge-test) of the European Pharmacopeia (EP). A D-optimal mixture design was used to provide a maximum of information from a limited number of experiments. The results of this study were analysed with the help of the Design Expert software and enabled us to formulate emulsions satisfying both requirements A and B of the EP.