Density functional theory calculations are presented for the linear cyano- and dicyanopolyacetylene cations C4N2+, C6N2+, HC3N+, and HC5N+. The structures, vibrational frequencies, and dipole moments are computed by unrestricted BLYP theory with the 6-311G* and 6-311G** basis sets. Calculated vibrational frequencies of the ground electronic states of the cations are in excellent agreement with experimental observations by Bondybey and co-workers and Maier and co-workers.