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The effect of three inert gases, helium, ethane, and pentene-1, on the rate of activation and reaction of methyl isocyanide in the low-pressure region at 280.5°has been examined. A study of the variation of the relative collision efficiency ß0(D) of these gases, as compared with the parent substrate, has been made as a function of dilution over the range D = -m. The predicted decrease of ß (0) with increasing dilution is observed. The variation is qualitatively correct in magnitude and increases as expected on going from the quasi-strong collider, pentene-1, to the weaker collider, helium. The measurements can in principle provide information about the nature of the collisional transition probability model(s) involved and the values of relative collision cross sections of inert molecules to substrate molecules. The present measurements are not sufficiently precise to permit full exploitation of this aspect but can be taken to suggest that a stepladder model fits best for pentene-1 and ethane, and an exponential model for helium. The average value of the energy decrement per collision experienced by the isocyanide molecule is of the order of 1.0, >5.0, and >6.0 kcal mole-1 for helium, ethane, and pentene-1, respectively.(1) (a) This work was supported by the National Science Foundation. (b) Abstracted from a Ph.D. thesis to be presented by Y. N. Lin which should be consulted for further details.(2) (a) H. S.