Abstract Rotational energy levels in the methyl cyanide molecule for rotational components 1 ≤ J ≤ 5 in the v 8 = 2 and v 8 = 3 vibrational states were studied for 13C-substituted species for a frequency range of 17 to 95 GHz. Anomalous behavior was observed in the v 8 = 3 vibrational states for the quantum number sets ( J, K = ±1, l = ±1), Kl = 1, with large departures between experimental and theoretical values. These departures are assumed to be due to Fermi resonance between the v 7 = 1 vibrational state and the v 8 = 3 vibrational level. Rotational components in each of these vibrations have been measured to determine if mutual interactions arise in them. The method of isotopic substitution enabled the molecular system to be internally “tuned” so that relative frequencies within each vibration with respect to another could be shifted. Thus, the v 8 = 3 vibrational level was lowered in frequency toward the v 7 = 1 vibrational level, bringing the two levels in question into nearer coincidence. In addition, experimental data are presented for rotational components in the v 4 = 1 vibrational state to test the hypothesis that the v 8 = 2 and the v 4 = 1 vibrational states are perturbing each other.