Abstract Cavity chamber absorbed dose measurements do not usually strictly adhere to the conditions of the Fano theorem and therefore the differences in the gas and wall mass stopping powers must be taken into account. Values of gas-to-wall absorbed dose conversion factors r m,g were calculated for neutron energies of 25 to 250 MeV for detectors with walls of C, O, Mg, Al, Si, Fe, Zr, AlN, Al 2 O 3 , SiO 2 , ZrO 2 , and A-150 tissue-equivalent (TE) plastic and with gas cavities of acetylene, dry air, Ar, an Ar–CO 2 mixture, CO 2 , isobutane, isobutane-based TE, methane, methane-based TE, propane, and propane-based TE. The r m,g calculations required initial spectral fluences of 1 H, 2 H, 3 H, 3 He, and 4 He ions released by neutron reactions in the walls, and these were calculated with the Los Alamos High Energy Transport code. Mass-stopping-power data were taken from Ziegler and co-workers. Additional calculations were made in order to test the sensitivity of r m,g to input data from other sources, i.e., ion spectral fluences from the ALICE nuclear reaction code and mass-stopping powers from the recent ICRU evaluation.