The 2'-deoxynucleoside containing the synthetic base 1-[(2R,4S,5R)-4-hydroxy-5-(hydroxymethyl)-tetrahydrofuran-2-yl)-1H-perimidin-2(3H)-one] (dPer) recognizes in DNA the O(6)-benzyl-2'-deoxyguanosine nucleoside (O(6)-Bn-dG), formed by exposure to N-benzylmethylnitrosamine. Herein, we show how dPer distinguishes between O(6)-Bn-dG and dG in DNA. The structure of the modified Dickerson-Drew dodecamer (DDD) in which guanine at position G(4) has been replaced by O(6)-Bn-dG and cytosine C(9) has been replaced with dPer to form the modified O(6)-Bn-dG:dPer (DDD-XY) duplex [5'-d(C(1)G(2)C(3)X(4)A(5)A(6)T(7)T(8)Y(9)G(10)C(11)G(12))-3']2 (X = O(6)-Bn-dG, Y = dPer) reveals that dPer intercalates into the duplex and adopts the syn conformation about the glycosyl bond. This provides a binding pocket that allows the benzyl group of O(6)-Bn-dG to intercalate between Per and thymine of the 3'-neighbor A:T base pair. Nuclear magnetic resonance data suggest that a similar intercalative recognition mechanism applies in this sequence in solution. However, in solution, the benzyl ring of O(6)-Bn-dG undergoes rotation on the nuclear magnetic resonance time scale. In contrast, the structure of the modified DDD in which cytosine at position C(9) is replaced with dPer to form the dG:dPer (DDD-GY) [5'-d(C(1)G(2)C(3)G(4)A(5)A(6)T(7)T(8)Y(9)G(10)C(11)G(12))-3']2 duplex (Y = dPer) reveals that dPer adopts the anti conformation about the glycosyl bond and forms a less stable wobble pairing interaction with guanine.