3,N4-Etheno, 3,N4-ethano, and 3-(2-hydroxyethyl)derivatives of 2'-deoxycytidine arise in mammalian DNA that has been exposed to the metabolic products of either vinyl chloride or the antitumor drug bis(chloroethyl)nitrosourea. These chemically-related adducts are thought to be associated with both mutagenesis and carcinogenesis. In this paper we report reliable syntheses of these deoxynucleosides and incorporation of the latter into oligodeoxynucleotides by the phosphoramidite route, using automated methods. It was found that 3-(2-hydroxyethyl)-2'-deoxycytidine is unstable in aqueous solution and undergoes an autoinduced hydrolysis to 3-(2-hydroxyethyl)-2'-deoxyuridine. The rate of this hydrolysis was found to be pH-dependent, having a maximum around pH 8, and a half-life of approximately 5 h. At higher or lower acidities, the reaction rate falls, indicating that the process involves a general acid-base catalysis. Thus in this case, oligomers were obtained that possessed 3-(2-hydroxyethyl)-2'-deoxyuridine residues, rather than the cytidine analogue. It is likely that the former represents the longer-lived species in DNA under physiological conditions. Representative oligomers containing these chemical lesions were analyzed by mass spectrometric and enzymatic degradation methods to confirm their structures.