It is shown that the cationic oligopeptides octadeca(L-lysine) (Lys18) and octadeca(L-ornithine) (Orn18) can induce a parallel duplex for the natural DNA oligomer dT10 with thymine-thymine base pairs. Complexation of the ammonium groups in the peptide side chains with the DNA phosphates leads to diminished electrostatic phosphate-phosphate repulsions, which allows this T-T base pair formation. From combined NOESY 1H NMR and molecular mechanics studies, it follows that the parallel duplex is right-handed, with the peptide located in the groove of the duplex. For the natural DNA oligomers dC10, d(C6T6), and d(T6C2T2), only Lys18 is able to induce the formation of parallel duplexes with C-C and T-T base pairs. It is shown that, for Orn18, a complexation must occur with one of the nonbonded oxygen atoms in the phosphate groups (OR) in such a way that unfavorable steric interactions are present with the C-C base pairs, which have a larger propellor twist angle than T-T base pairs. An analogy is presented between peptide complexation with the phosphates and the neutralization of the phosphate groups by methylation, which is known to lead to parallel duplexes with T-T base pairs (for both the Sp and Rp configurations) and C-C base pairs (only for the Sp configuration).