Structures, bonding and relative stabilities of the radical dimer cations (XH3-YH3)1(X,Y=N, P, As) have been studied theoretically. Two kinds of structures (hemi-bonded and proton-transferred isomers) are obtained for each system. For (NH3-N/P/AsH3)(+) the stable conformer is the proton-transferred structure; for (PH3-PH3)(+) and (AsH3-AsH3)(+) the stable structure is the hemi-bonded one; for (PH3-AsH3)+ three proton-transferred and one hemi-bonded isomers were found with the stability order: (H-PH3+ ... AsH2) I> (H3PH+ ... AsH2) II > (H3P ... AsH3+) > (H3AsH+ ... PH2) III. The hemi-bonds have large interaction energies 25.2-35.1 kcal/mol and are partially covalent in nature, while the proton-transferred structures have moderate interaction energies 6.5-22.2 kcal/mol. (C) 2014 Elsevier B.V. All rights reserved.