Objectives: Previous research on a group of workers occupationally exposed to styrene, ethylene glycol, toluene, p-xylene and their mixture showed elevated levels of the main products of lipid peroxidation: malondialdehyde and 4-hydroxynonenal (MDA+,4-HNE) in plasma [1]. Moreover, an earlier in vitro study indicated a synergistic interaction between styrene and ethylene glycol on lipid peroxidation [2]. Therefore, it seemed interesting to investigate the effect of combined exposure to toluene and p- xylene on lipid peroxidation and define the type of the interaction. Materials and Methods: An in vitro model of human placenta mitochondria was used in the study. The concentration of TBARS (thiobarbituric active reagent species) was measured by spectrophotometry, and of hydroxyl radical (•OH) by assessment of deoxyribose degradation. It was investigated whether the administration of coenzyme Q10 (CoQ10) could have a protective function (if given before solvent exposure) or a reparatory function (if given after exposure) in solvent-induced oxidative stress. Results: Exposure to p-xylene at concentrations ranging from 5.3 to 265 μg/ml produced an increase in TBARS concentration. The results showed that p-xylene had a stronger influence on lipid peroxidation than toluene. The mixture of toluene and p-xylene induced an antagonistic effect on lipid peroxidation, measured as TBARS concentration. The mechanism connected with •OH generation was found to play an important role in the oxidative damage to lipids resulting from p-xylene exposure. Administration of coenzyme Q10 at the doses of 3.0 and 12.0 μg/ml successfully decreased the TBARS level that was elevated after solvent exposure. Conclusions: In contrast to the synergistic effect that the mixture of styrene and ethylene glycol had on lipid peroxidation (previous study), an antagonism between toluene and p-xylene could be observed. The coenzyme Q10 can be considered a protective agent against lipid peroxidation.