The thermal isomerisation of methyl isocyanide has been measured in the rigorous absence of self-heating over a temperature range from 120–320 °C and at pressures from 2–100 torr. Expressions are given for the rate constant as functions of both temperature and pressure in these ranges of the two variables. The infinite-pressure rate constant is accurately represented by an Arrhenius line with no curvature having E∞ = 38.2 ± 0.2 kcal mol−1 and log10A∞ (s−1) = 13.35 ± 0.11, although at all other pressures, E is a function of temperature.