Analysis of the reaction between 2'-deoxyadenosine and 13-hydroperoxylinoleic acid by liquid chromatography/constant neutral loss mass spectrometry revealed the presence of two major products (adducts A and B). Adduct A was shown to be a mixture of two isomers (A1 and A2) that each decomposed with the loss of water to form adduct B. The mass spectral characteristics of adduct B were consistent with the substituted 1,N6-etheno-2'-deoxyadensoine adduct 1'-[3-(2'-deoxy-β-D-erythro-pentafuranosyl)-3H-imidazo[2,1-i]purin-7-yl]hept an-2'-one. Adducts A1, A2, and B were formed when 2'-deoxyadenosine was treated with synthetic 4-oxo-2-nonenal, which suggested that it was formed by the breakdown of 13-hydroperoxylinoleic acid. A substantial increase in the rate of formation of adducts A1, A2, and B was observed when 13-hydroperoxylinoleic acid and 2'-deoxyadenosine were incubated in the presence of Fe(II). Thus, 4-oxo-2-nonenal was most likely formed by a homolytic process. Although adducts A1, A2, and B were formed in the reaction between 4-hydroxy-2-nonenal and 2'-deoxyadenosine, a number of additional products were observed. This suggested that 4-hydroxy-2-nonenal was not a precursor in the formation of 4-oxo-2-nonenal from 13-hydroperoxylinoleic acid. This study has provided additional evidence which shows that 4-oxo-2-nonenal is a major product of lipid peroxidation and that it reacts efficiently with DNA to form substituted etheno adducts.