Diesel exhaust particles (DEP) contain quinones that are capable of catalyzing the generation of reactive oxygen species in biological systems, resulting in induction of oxidative stress. In the present study, we explored sulfhydryl oxidation by phenanthraquinone, a component of DEP, using thiol compounds and protein preparations. Phenanthraquinone reacted readily with dithiol compounds such as dithiothreitol (DTT), 2,3-dimercapto-1-propanol (BAL), and 2,3-dimercapto-1-propanesulfonic acid (DMPS), resulting in modification of the thiol groups, whereas minimal reactivities of this quinone with monothiol compounds such as GSH, 2-mercaptoethanol, and N-acetyl-L-cysteine were seen. The modification of DTT dithiol caused by phenanthraquinone proceeded under anaerobic conditions but was accelerated by molecular oxygen. Phenanthraquinone was also capable of modifying thiol groups in pulmonary microsomes from rats and total membrane preparation isolated from bovine aortic endothelial cells (BAEC), but not bovine serum albumin (BSA), which has a Cys34 as a reactive monothiol group. A comparison of the thiol alkylating agent N-ethylmaleimide (NEM) with that of phenanthraquinone indicates that the two mechanisms of thiol modification are distinct. Studies revealed that thiyl radical intermediates and reactive oxygen species were generated during interaction of phenanthraquinone with DTT. From these findings, it is suggested that phenanthraquinone-mediated destruction of protein sulfhydryls appears to involve the oxidation of presumably proximal thiols and the reduction of molecular oxygen.