Gas-to-wall absorbed dose conversion factors rm.g were calculated with Bragg_Gray cavity theory for the 25 to 250 MeV neutron energy range. Calculations of rm.g were made for propane-based tissue-equivalent gas and walls of C, O, Mg, Al, Si, Fe, Zr, AlN, Al2O3, Zr, ZrO2, and A-150 plastic. Charged particle production in the wall materials was calculated with nuclear model codes. Mass stopping powers were taken from recent tabulations. Above 70 MeV neutron energy the rm.g values were found to be nearly constant and to approach the wall-to-gas mass stopping power ratio of minimum ionising particles. Slight energy dependencies were found in rm.g below 70 MeV. Uncertainties in rm.g are estimated to be less than five per cent below 100 MeV neutron energy and are primarily from the uncertainties of the stopping power data.