1. Reactivity of benzene oxide (BO), a reactive metabolite of benzene, was studied in model reactions with biologically relevant S- and N-nucleophiles by LC-ESI-MS. 2. Reaction with N-acetylcysteine (NAC) in aqueous buffer solutions gave N-acetyl-S-(6-hydroxycyclohexa-2,4-dien-1-yl)cysteine (pre-phenylmercapturic acid, PPhMA), which was easily dehydrated in acidic solutions to phenylmercapturic acid (PhMA). The yield of PPhMA + PhMA increased exponentially with pH up to 11% in the pH range from 5.5 to 11.4. 3. Primary 6-hydroxycyclohexa-2,4-dien-1-yl (HC) adducts were detected also in reactions of purine nucleosides and nucleotides under physiological conditions. After a vigorous acidic hydrolysis, all HC adducts were converted to corresponding phenyl purines, which were identified as 7-phenylguanine (7-PhG), 3-phenyladenine (3-PhA) and N(6)-phenyladenine (6-PhA). The yield of 7-PhG amounted to 14 ± 5 and 16 ± 7 ppm for 2'-deoxyguanosine and 2'-deoxyguanosine-5'-monophosphate, respectively, that of 6-PhA was 500 ± 70 and 455 ± 75 ppm with 2'-deoxyadenosine and 2'-deoxyadenosine-5'-phosphate, respectively, with only traces of 3-PhA. 4. Reactions with the DNA followed by acidic hydrolysis yielded 26 ± 11 ppm (mean ± SD; n = 9) of 7-PhG as the sole adduct detected. 5. In contrast to the reactions with S-nucleophiles, the reactivity of BO with nucleophilic sites in the DNA is very low and can therefore hardly account for a significant DNA damage caused by benzene.