Thermal explosion limits have been measured in the temperature range 285–351 °C for methyl isocyanide and at 351 °C for mixtures of methyl isocyanide and methyl cyanide. We suggest that the methyl isocyanide isomerization is potentially the best reaction system against which to test thermal explosion theories, but at the present time certain kinetic, thermochemical, and thermodynamic data are not available with sufficient accuracy.