We have extended to higher frequency our earlier assignments (J. Mol. Spectrosc.162,142–151, 1993) of the dense rotation–torsion structure in theGsmethyl rocking infrared fundamental band of dimethylacetylene (CH3CCCH3or 2-butyne) centered at about 1036 cm−1. TheE2dcombination band ν4+ ν16at 1057 cm−1significantly perturbs this high-frequency structure, and we include its effect explicitly in the Hamiltonian matrix. We can now include considerably more of the high-frequency data in the analysis, and the new fitting leads to a determination of the ground state torsional potential parameterV3as 1.517 cm−1, and a torsional barrier height (4V3+ 4V9) of 5.98 ± 0.03 cm−1. We cannot reproduce satisfactorily the structure in the immediate vicinity of the perturbation because of our lack of data for the ν4+ ν16perturbing state.