When a Ne:C 2H 2 sample is codeposited at approximately 5 K with a beam of neon atoms that has been excited in a microwave discharge, a sharp, prominent absorption assigned to ν3 of HCCH + appears at 3137.6 cm -1, very close to the previously reported gas-phase band center. Experiments on carbon-13 and deuterium substituted samples support this assignment and permit the identification of all of the infrared-active CH- and CD-stretching fundamentals of the isotopically substituted acetylene cations, as well as the determination of the stretching and stretching-interaction force constants. The absorptions of the carbon-13 substituted acetylene cations have also been identified in the analogous argon-matrix experiments, but exhibit a matrix shift of approximately 30 cm -1, possibly because of the larger polarizability of argon. The ν3 absorption of H 13C 13CH + isolated in a neon matrix is split by interaction with a combination band, probably ν2 + ν4 + ν5 (Σ u+). The CC-stretching fundamental of HCC - has been tentatively identified at 1773.0 cm -1, and the band origin of the A2Π u- X2Σ g+ transition of C 2- near 4155 cm -1, in neon-matrix experiments.