MP2/6-311++G(3df,3pd) calculations were performed on C2H3+ and C2H5+ three-center, two electron (3c-2e) systems with T-shaped approach of proton to acetylene and ethylene, respectively. The Quantum Theory of Atoms in Molecules (QTAIM) was applied to analyze these moieties. It was found that H+ ... pi interactions in C2H3+ and C2H3+ possess the characteristics typical for covalent bonds. Two complexes: C2H5+ ... C2H2 and C2H3+...H-2 were also considered where C(2)H(5)(+)and C2H3+ act as proton donating systems while C2H2 and H-2 as proton acceptors, respectively - these complexes possess the characteristics of H-bonded species. (c) 2007 Elsevier B.V. All rights reserved.