The population relaxation rate of the first excited state of the acetylenic C-H stretch is compared for room-temperature gas-phase and solution-phase samples of 10 terminal acetylenes. The gas-phase sample pressure is less than 1 atm for all measurements, ensuring that the dynamics occur under collision-free conditions. The relaxation rates are measured using two-color transient absorption picosecond spectroscopy. The population of the excited state is monitored directly through the anharmonically shifted v = 1 - v = 2 excited-state transition. The relaxation rates of isolated and solvated molecules are strongly correlated and follow a relationship expected for a parallel relaxation process in solution: the total rate in solution is the sum of a molecule-dependent rate related to the isolated molecule dynamics and a molecule-independent solvent-induced relaxation rate. For the terminal acetylenes, the vibrational normal-mode frequencies of the acetylene chromophore and their anharmonic interactions are highly conserved for all terminal acetylenes. Therefore, the observation that a single solvent relaxation contribution to the total relaxation rate describes the solution dynamics for all terminal acetylenes is consistent with the idea that solvent-induced energy relaxation pathways are dominated by the vibrational motions that are in close proximity to the excited state.