Abstract Vibration-rotation bands of methyl acetylene have been re-investigated with high resolving power. Five parallel bands (ν 1, ν 2, ν 5, 2ν 9 and 2ν 8) have been analysed, and values have been derived for α A and α B relating the rotational constants A and B in different vibrational states. The rotational structure of four degenerate fundamental vibration bands ν 6, ν 7, ν 8 and ν 9 has been analysed. Perturbations of ν 7 by Coriolis interaction with ν 7, and Fermi resonance with (ν 8 + ν 10) have been detected, and analysed in the latter case. An unusual rotational structure of ν 9 has been found, arising from the particular value of ζ, the Coriolis coefficient, and relative values of A and B. The earlier data on ν 10 of Rao and Palik have been re-examined and interpreted in a different way. Using all the data obtained, with the sum rule, values of ζ 6 = 0·076, ζ 7 = −0·339, ζ 8 = 0·397, ζ 9 = 1·00, ζ 10 = 0·89, and A 0 = 5·31 cm −1 were determined.