In ionography the x‐ray photons are absorbed in a high‐pressure gas between the parallel plates of an ion chamber and the ions produced are collected on an insulating foil covering one of the plates. The charge pattern is rendered visible using powder‐cloud development. The effects of plate separation and the nature of the gas on the images produced by x rays in the range 50 to 105 kVp have been investigated both experimentally and theoretically. Three potentially useful gases (freon 13B1, krypton, and xenon) have been studied. The energy spectrum of electrons set in motion in these gases have been calculated and the absorption efficiency determined. For a 2‐cm gap with a pressure of 10 atm using x rays at 45 kVp all three gases give an efficiency of about 80% while at 105 kVp the efficiencies are 62%, 28%, and 26% for xenon, krypton, and freon, respectively. Higher efficiencies can be obtained using larger gaps. The effects of gas, gas pressure, plate separation, and x‐ray spectrum on the resolving capability were calculated. Freon gives about twice the resolution of either krypton or xenon. The resolution was measured and found to be greater than 10 line pairs per mm in freon at 10 atm. Diffusion does not blur the image appreciably when positive or negative ions are collected but is important when electrons are used to form the image. Fields of the order of 25 kV/cm are required to collect 90% of the ions at exposure rates of 25 mR/sec. Our results suggest that with further technical development ionography may offer some real advantages in diagnostic radiology.