Abstract An approach to evaluate preservative efficacy using a 23 factorial design is presented. The influence of pH and tonicity of the solution and the presence of EDTA on the antimicrobial activity of phenol was studied. C. albicans and E. coli were employed as test organisms. Statistically significant effects ( P < 0.005) of pH and the presence of EDTA on the efficacy of phenol were obtained with both organisms. The tonicity of the solution had a significant effect (if P < 0.005) only with E. coli. Significant interactions ( P < 0.005) were found between pH and tonicity of the solution with both organisms, and between tonicity of the solution and the presence of EDTA only with C. albicans. Furthermore, interactions among all 3 factors were detected with C. albicans (P < 0.005). The application of a factorial design study can be useful in the evaluation of preservative efficacy at the preformulation stage.