The deoxyadenosine derivative tethering the phenyl group at N6 of deoxyadenosine (A(phe)) was previously found to have a property to stack strongly with adjacent nucleotide bases in a DNA duplex. On the other hand, it was also demonstrated that DNA polymerases selectively incorporated dTTP opposite A(phe) in a template DNA strand. These observations suggest that the conformation of A(phe) in solution differs from that during the DNA polymerase reaction. Here, the chemical modifications of thymine bases in a DNA duplex by KMnO(4) and CMCT (1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide metho-p-toluene sulfonate) were examined, and it was revealed that the thymine base opposite A(phe) was efficiently flipped out of the DNA helix as much as that in a single-stranded DNA.