The stabilities of products for proton-transfer reactions in stacked two guanine(G)-cytosine(C) pairs are discussed by means of Hartree-Fock, MP2 and density functional methods. The stacking energy of the original 5'-d(CpG)-3' (CG/GC) differs from that of single proton-transfer (SPT) product. On the other hand, the stacking energy of the original 5'-d(GpG)-3' (GC/GC) is essentially equal to the double proton-transfer (DPT) products. The dispersion forces of both CG/GC and its SPT product are similar to each other. Not only dispersion force but also Coulomb attraction contributes to the stability of the SPT product of CG/GC. (C) 2009 Elsevier B. V. All rights reserved.