Traditionally, air- and tissue-equivalent ion chambers have been used to determine the strength of radiation fields or the dose deposited in tissue, independent of the gamma-ray energy involved. Some applications, including those associated with U.S. Nuclear Regulatory Commission Regulatory Guide 1.97, call for the use of metallic-walled chambers with such an energy-independent response requirement. We examined by means of calculations and measurements the effect of three different gas fills on this characteristic of a stainless steel/aluminum Westinghouse ion chamber. Details of the gamma ray and electron interactions in these chambers have been examined and they have shown that both secondary interactions of scattered gamma rays and interactions of the incident gamma rays directly with the chamber gas are important current-producing mechanisms in ion chambers with large electrode structures. This information, along with results concerning the effect of gas pressure on the energy loss of traversing electrons, is used to explain the calculated flat energy response from a 1-atm 95% N—5% He gas fill, the less flat response for a 10-atm fill of the same gas, and the strong energy dependence of 7.5 atm of xenon.