The effects of phenolic acids of microbial origin on mitochondrial functions and the possibility of removing their effects by thiol antioxidants dithiotreitol and N-acethylcysteine were studied. The action of some phenolic acids on the redox state of NADH, the membrane potential and calcium capacity of mitochondria is due to their interaction with thiol groups. The partial restoration of mitochondrial functions occurred in the presence of dithiotreitol and N-acethylcysteine, the full recovery (short-term duration) was promoted by the combined action of dithiotreitol and menadione (vitamin K3). It was found that the protective effect of thiol antioxidants became prooxidant one, if the medium contained free iron and compounds with a quinone structure, capable of entering into a redox cycle with thiols. It is shown that the interaction of thiols with iron and menadione is accompanied by absorption of oxygen to form superoxide anion. Prooxidant effect of thiol antioxidants may explain the absence of the protective effect at the later stages of sepsis and systemic inflammatory syndrome.