Evidence is presented that shows very slow intramolecular energy redistribution (IVR) in the acetylenic C-H stretch fundamental of molecules in planar conformations. Each molecule,4-chlorobut-1-yne, 4-bromobut-1yne, 4-fluorobut-1-yne, methyl propargyl ether, 1-pentyne, and pent-1-en-4-yne have a planar trans conformer (except for pent-1-en-4-yne where the planar conformer is cis) that is connected to a nonplanar gauche (skew) conformer by rotation about a C-C bond (C-O for methyl propargyl ether). The planar forms (observed for each molecule except methyl propargyl ether) of these molecules exhibit some of the slowest hydride stretch IVR rates measured, with τ IVR for the acetylenic C-H stretch ranging from 1 to 3 ns. These results are compared to the IVR lifetimes for the nonplanar conformers of 1-pentyne and methyl propargyl ether (τ IVR ≈ 300 ps). Also presented are a series of molecules ((Z)-pent-3-en-1-yne, (E)-pent-3-en-1-yne, 1-butyne, 3-fluorobut-1-yne, and 2-methyl-1-buten-3-yne) that have a single methyl group substituent and do not form different conformations upon rotation about the C-C bond. The barrier to internal rotation of the methyl tops range from 389 to 2016 cm -1 ; however, the IVR lifetimes of the acetylenic C-H stretch for each is near 100 ps.