N-Nitrosopiperidine, a carcinogenic cyclic nitrosamine that occurs in the diet and may be formed endogenously, is believed to be metabolically activated by α-hydroxylation. The DNA reactive compounds that could be formed in this process have been studied using α-acetoxy-N-nitrosopiperidine as a model. Previous studies have shown that 4-oxo-2-pentenal is one product of the hydrolysis of α-acetoxy NPIP and that it reacts with deoxyguanosine to produce 7-(2-oxopropyl)-5,9-dihydro-9-oxo-3-β-D-deoxyribofuranosylimidazo[1,2- α]purine(7-(2-oxopropyl)-1,N2-ethenodG). Several other products were formed in that reaction, and these have now been identified as diastereomers of 7-(2-oxopropyl)-5-hydroxy-5,6,7,9-tetrahydro-9-oxo-3-β-D- deoxyribofuranosylimidazo[1,2-a]purine, the hemiaminal precursors to 7-(2-oxopropyl)-1,N2-ethenodG. Their structures were characterized by electrospray ionization mass spectrometry (ESI-MS), and by reduction with NaBH4 followed by hydrolysis to 7-(2-hydroxypropyl)-5,6,7,9-tetrahydro-9-oxoimidazo[1,2-a]purine, which was characterized by 1H-NMR, MS, and UV. The presence of 7-(2-oxopropyl)-1,N2-ethenodG and its hemiaminal precursors in DNA reacted with either α-acetoxy-N-nitrosopiperidine or cis-4-oxo-2-pentenal was confirmed by LC-ESI-MS and LC-ESI-MS/MS. The results of this study demonstrate that ethenodG adducts and their precursors are present in DNA reacted with α-acetoxy-N-nitrosopiperidine, which suggests a possible basis for the unique carcinogenic properties of this nitrosamine.