The method of obtaining collisional energy transfer data for collisions between a highly excited polyatomic and a bath gas from classical trajectories is discussed. The technique yields the mean(-square) energy transferred per collision from a comparatively small (≊500) number of trajectories. Results compare favourably, qualitatively and quantitatively, with experiment for all except the lightest bath gases (where the interaction potential is uncertain). Simple analytical models are also examined. These yield methods of obtaining energy transfer data of practical use in predicting and interpreting falloff data for unimolecular and recombination reactions. It is shown that quantum effects are unimportant for such systems.