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Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C 4 H 4•+ ion in the cluster beam. The measured average collision cross section of the C 4 H 4•+ isomers in helium (38.9±1 Å 2) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C 4 H 4•+ ion [methylenecyclopropene (39.9 Å 2), 1, 2, 3-butatriene (41.1 Å 2), cyclobutadiene (38.6 Å 2), and vinyl acetylene (41.1 Å 2)]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C 4 H 4•+ ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained …