Hypochromic effects of 2-(thymin-1-yl)propionyl graft polyethylenimine (P-T) and its related monomer (M-T) and dimer model (D-T), and 2-(adenin-9-yl)propionyl graft polyethylenimine (P-A) and its monomer (M-A) and dimer model (D-A) have been systematically investigated, and the base-stacked conformation in comparison with corresponding polynucleotides and dinucleotides has been discussed. The results suggest that the graft polymers and even their dimer models may have a particular preference for a stacked conformation compared with the corresponding poly- and dinucleotides. A study of solvent effect on the NMR spectra of the dimer model suggests the existence of a preferred base-stacked conformation. Base pairing effects between the complementary bases have also been studied for various combinations among the model systems. Only the combination P-T/P-A showed a considerable hypochromic effect.