The aim of the present study is to determine the chemical structure and conformation of DNA adducts formed by incubation of the bioactive form of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), N-acetoxy-PhlP, with a single-stranded 11mer oligodeoxyribonucleotide. Using conditions optimized to give the C8-dG-PhlP adduct as the major product, sufficient material was synthesized for NMR solution structure determination. The NMR data indicate that in duplex DNA this adduct exists in equilibrium between two different conformational states. In the main conformer, the covalently bound PhlP molecule intercalates in the helix, whilst in the minor conformation the PhlP ligand is probably solvent exposed. In addition to the C8-dG-PhIP adduct, at least eight polar adducts are found after reaction of N-acetoxy-PhlP with the oligonucleotide. Three of these were purified for further characterization and shown to exhibit lowest energy UV absorption bands in the range 342-347 nm, confirming the presence of PhlP or PhlP derivative. Accurate mass determination of two of the polar adducts by negative ion MALDI-TOF MS revealed ions consistent with a spirobisguanidino-PhlP derivative and a ring-opened adduct. The third adduct, which has the same mass as the C8-dG-PhlP oligonucleotide adduct, may contain PhlP bound to the N2 position of guanine.