The average energy expenditure per ion pair for carbon and nitrogen recoils (W_r) has been compared with that for ²³⁸Pu alpha particles (W_α). The pulse heights of the recoil and alpha particles were compared in a proportional counter, the recoils being produced by the interaction of 2·9 MeV neutrons with the gas in the counter. The ratio of W_r to W_α has been found to be between 1·2 and 1·3 for carbon recoils in carbon dioxide, ethylene, methane, and tissue equivalent gas, and to be 1·2 for nitrogen recoils in nitrogen gas.