Aims: The aim of the study was to verify the antimicrobial activity of commercial essential oils: lavender, tea tree and lemon as the components of a preservative system in oil in water body milks. Methods and Results: The inhibition efficacy of essential oils alone (0 center dot 5%), in mixtures (1%) as well as combined with the synthetic preservative 1,3-dimethylol-5,5-dimethylhydantoin and a 3-iodo-2-propynyl butyl carbamate mixture (0 center dot 1% and 0 center dot 2%) was tested against Staphylococcus aureus ATCC 6538, Pseudomonas aeruginosa ATCC 9027, Candida sp. LOCK 0008 and Aspergillus niger ATCC 16404 in compliance with the standards of the European Pharmacopoeia Commission. The in vitro activity of oils determined by an impedimetric method was also compared with their activity in cosmetic preparations. Criterion A for bacteria (reduction in the inoculum by 3 logarithmic units within 7 days with no increase up to the 28th day) and fungi (reduction in the inoculum by 2 logarithmic units within 14 days with no increase up to the 28th day) was fulfilled for cosmetic formulations containing the tested essential oils with 0 center dot 2% of the synthetic preservative. The preservative concentration could be decreased to 0 center dot 1% (with preserving the same efficacy) in combination with lavender and tea tree oils at a concentration of 0 center dot 5% each. Conclusions: In all combinations of essential oils with the synthetic preservative, a synergistic effect of the preservative system components was observed, which made it possible to reduce the usable level of the synthetic preservative up to 8 center dot 5 times. Significance and Impact of the Study: To develop an effective preservative system in cosmetics in which a synthetic chemical preservative is replaced by natural essential oils.