The Fermi and Coriolis interacting band system ν 4, ν 7, ν 8 + ν 10 of methyl acetylene has been investigated with a resolution better than 0.3 cm −1, and a previous analysis of Thomas and Thompson shown to be in error. The band system is complicated by a very localized perturbation, almost certainly with ν 5 + ν 9, at ∼1540 cm −1, and by a series of hot transitions. We have been able to identify Q branches arising from both the main components, ν 7 and ν 8 + ν 10, throughout the band system, and analysis of these leads to the molecular parameters: ν 7 0 = 1450.88 cm −1 , ζ 7 z = −0.311 , (α 7 A - α 7 B) = +0.053 cm −1 , ( ν 8 + ν 10) 0 = 1371.08 cm −1, ζ eff z = −1.296, ( α 8, 10 A - α 8, 10 B ) = −0.035 cm −1, k 7810 = 25.3 cm −1. The veracity of this analysis is confirmed by computation of the theoretical band contour with the above constants. This enables the unperturbed vibrational transition moment ratio M 7: M 8,10 to be determined to be 1 : 0.05± 0.01, and shows that the intensity perturbation due to the Fermi resonance is a negative one, i.e., the triple product M 7· M 8,10· k 7810 is negative. By further taking the A 1 - E Coriolis interaction between ν 4 and ν 7 into account without approximation, it is shown that the complicated structure in the region of 1390 cm −1 is due to R Q 0 of the lower Fermi component, P Q 5 of the upper component, heavily degraded to high wavenumbers by the Coriolis interaction, and the Q branch of ν 4 with its attendant unresolved P and R branches. Exact computations taking into account the Coriolis interaction enables the further molecular parameters to be determined: ν 4 0 = 1390.55 ± 0.2 cm −1 , | ζ 4,7 a y | = 0.55 ± 0.05. The best fit with the observed spectrum is obtained with an unperturbed ratio of vibrational transition moments M 7 : M 4 = 1 : 0.4 ± 0.1, and with a negative intensity perturbation due to the Coriolis interaction, i.e., the triple product M 4 z·M 7a x·ζ 4,7a y is negative. In the course of this analysis, a mis-assignment by three units in K was suspected in the very weak perpendicular fundamental ν 8, and is supported by overtone and combination data. As a consequence, the band center of ν 8 of methyl acetylene is revised from the original value of 1053.2 cm −1 to the new value of 1036.03 cm −1. The slight intensity asymmetry in this band almost certainly occurs through Coriolis interaction with ν 7 giving rise to a positive intensity perturbation.