A newly developed method is used to calculate the average energy transferred in collisions between a highly vibrationally excited methylperoxy radical and argon bath gas. The method involves modelling the process through classical trajectory simulations with accurate intra- and inter-molecular potentials. These calculations show that the root-mean-squared internal energy transferred per collision is c. 275 cm-1 for 300 ≤ T/K ≤ 600 (the CH302 internal energy being 104 cm-1, the dissociation threshold), while the same quantity for rotational energy is c. 290 cm-l. These results make it possible for rate data obtained by other workers for the reaction CH3O2+M ↔ CH3+02+M over limited pressure and temperature ranges to be used to predict reliably the appropriate rate coefficients at any temperature and pressure.