Antioxidant activities of essential oil mixtures--thyme or clove leaf with cinnamon leaf, rose, or parsley seed--toward skin lipid, squalene oxidized by UV irradiation were investigated using the malonaldehyde/gas chromatography assay. At all concentrations (50, 100, or 500 mug/mL) tested, thyme oil mixed with 500 mug/mL clove oil showed over 90% inhibitory effect against malonaldehyde formation. The order of potency of all oils mixed together at 500 mug/mL was thyme/clove leaf (93%) > clove leaf/parsley seed = clove leaf /rose (87%) > thyme/parsley seed (83%) > clove leaf/cinnamon leaf (77%) > thyme/parsley seed (71%) > thyme/cinnamon leaf (7%). In comparison, the inhibitory activities of 500 microg/mL of BHT or alpha-tocopheroltoward malonaldehyde formation were 85% and 76%, respectively. Pro-oxidant effects were observed for some mixtures of thyme with cinnamon leaf or rose oils. The potent antioxidant effects resulting from a mixture of thyme and clove leaf oils may be due to the presence of thymol and eugenol.