The bay region epoxide of benzo[a]pyrene (anti-BPDE) alkylates DNA to form adducts with >98% trans stereochemistry. Halide ions catalyze this reaction; however, this pathway is characterized by the formation of adducts with altered cis stereochemistry. Bay region halohydrins are possible intermediates in these reactions, but are too unstable to be isolated from aqueous solutions. However, we successfully synthesized halohydrins in tetrahydrofuran (THF) by treatment of anti-BPDE with the corresponding lithium halide salt in the presence of acetic acid. Absorbance and CD spectroscopy clearly indicated the formation of chloro-, bromo-, and iodohydrins. The structure and stereochemistry of the chlorohydrin was established by NMR. Chloride addition is exclusively at the benzylic position of the epoxide, and the stereochemistry of the C-9 and -10 positions is trans. The long-wavelength absorbance band in the chloro-, bromo-, and iodohydrin is red-shifted 7, 13, and 22 nm, respectively, relative to the hydrolysis product of anti-BPDE. The ellipticity of the same absorbance band in CD spectra of enantiomerically pure halohydrins is opposite in sign compared to that of the corresponding anti-BPDE enantiomer. The relative stability of these derivatives is chlorohydrin > bromohydrin > iodohydrin. The chloro- and bromohydrins were isolated, but the iodohydrin decomposed during this manipulation. The addition of 500 mM chloride decreased the hydrolysis rate of the chlorohydrin 4-fold in 50% THF/buffer. Direct evidence for the transient formation of the iodohydrin in aqueous buffer/acetone mixtures was obtained by absorbance spectroscopy. At 1 M chloride, bromide, and iodide, alkylation of deoxyadenosine by anti-BPDE in aqueous buffer yields 85, 91, and 92% cis adducts, respectively. In the absence of halide, alkylation of deoxyadenosine in buffer by anti-BPDE, the chlorohydrin, and the bromohydrin yields 32, 65, and 83% cis adducts, respectively.