The reaction of the ethylene radical cation (Et.+) with acetylene (Ac) to form stable C4H6.+ intermediates and the subsequent fragmentation of these to C3H3+ + CH3. or to C4H5+ + H-. have been studied by the UMP2, RMP2, and B3LYP methods with the 6-31G* basis set, as well as by single-point calculations at the RCCSD-(T)/cc-pVTZ level of theory. The aim of this study was to identify all stationary points that might be relevant to explain the course of the observed reactions. According to their stability to dissociation, we distinguish three classes of C4H6.+ structures : weakly bonded complexes, structures of medium stability, and tightly bonded complexes. Methylcyclopropene radical cation seems to be the most likely ultimate precursor for the formation of the observed fragmentation products.