Triclosan although banned for the use in many cosmetic products is still used in endodontic therapy for the elimination of microorganisms. At minimum inhibitory concentrations (MIC), triclosan is bacteriostatic and selects resistant clones with prolonged exposure. This study aims to demonstrate the use of eugenol microemulsions as a vehicle to improve the activity of triclosan and prevent the selection of resistant clones. The solubility of triclosan in various oils (cinnamon oil, clove oil, eugenol and peppermint oil) and the release kinetics of triclosan from the microemulsions was determined spectrophotometrically at 281 nm. Microemulsions were formulated by the water titration method. The MIC and minimum bactericidal concentration (MBC) were determined by the broth microdilution method. Triclosan resistant clones were selected by serial passage method. Triclosan has 4 fold difference between its MIC and MBC against S. aureus, E. faecalis and S. mutans isolates. The triclosan loaded microemulsion (5%:25%:70% ratio of oil, Tween 20 & water) (EuTT20-5), containing a final concentration of 3 mg ml−1 of triclosan, eliminated this difference. On repeated passages with increasing concentrations of triclosan, mutant strains of E. faecalis with 8-fold increase in MBC (250 μg ml−1 to 2 mg ml−1) and mutant strains of S. aureus and S. mutans with 8-fold increase in MBC from 125 μg ml−1 to 1 mg ml−1 were observed. Such mutants were not generated in the EuTT20-5 treated cultures. The resistant mutants demonstrated only a 2-fold increase in MBC of EuTT20-5 which is insignificant. Thus, EuTT20-5 can not only improve the efficacy of triclosan, but it can act with equal potency against triclosan resistant isolates. Eugenol loaded triclosan microemulsions can be used instead of conventional biocidal solutions in endodontic therapy where complete elimination of microorganisms from the treatment site is required. © 2020 Elsevier B.V.