Cyanoacetylene dimer cations, (HC3N)(2)(+), were produced in a selected-ion flow tube in helium buffer at 0.35 +/- 0.1 Torr and 294 +/- 3 K either directly by the association of HC3N+ with HC3N or indirectly in a sequence of reactions of C-60(2+) With two molecules of HC3N. Two distinctly different isomers were identified by multiple collision-induced dissociation. The structures and energies of nine isomers of (HC3N)(2)(+) were computed at the B3LYP/6-31+G(d) and B3LYP/6-311++G(2df,p) levels together with selected energies of dissociation. A comparison of the latter with observed dissociation pathways and onset energies indicates that the (HC3N)(2+) formed directly is a solvated ion and that the (HC3N)(2)(+) produced in the presence of C-60(2+) is a cyclic dicyanocyclobutadiene cation. A mechanism is proposed for the formation of the cyclic isomer that involves a 2 + 2 cycloaddition of HC3N to the charged terminus of a cyanoacetylene molecule anchored to C-60 through a C-N bond. The results point toward a general gas-phase reaction route for the cyclization of cyanoacetylenes in the presence of doubly charged substrates and have implications for the dimerization of HC3N and higher members of the homologous series of cyanopolyacetylenes in interstellar/circumstellar environments.