In order to determine the intensity of Xor gamma radiation by means of an ionization chamber, it is necessary to obtain a theoretical relation between the radiation intensity incident on the chamber and the ionization produced in the gas volume of the chamber by the secondary electrons set in motion by the radiation. This relation will, in general, be a function of the energy of the incident radiation, the nature and pressure of the gas in the chamber, and the thickness and composition of the chamber walls. Theoretical expressions relating radiation flux and chamber ionization have been developed by Laurence (1), Lax (2), Fowler et al. (3), Flowers et al. (4), Spicer (5), and others. However, each of these authors has considered ionization chambers made of one material only, whereas in the present work the dependence of ionization upon the atomic number of the chamber wall has been investigated. A number of the simplifying assumptions made in previous work have been avoided. In the following discussion, the problems of relating the incident radiation intensity to the measured ionization current has been considered in two parts: (1) an expression has been obtained for the ionization produced in a small gas-filled cavity surrounded by homogeneous material irradiated with monochromatic radiation of uniform intensity throughout the material. (2) A correction to this expression has been applied to allow for the fact that in an actual chamber the radiation intensity is attenuated in passing through the chamber walls. The solution of the second problem requires a knowledge of the variation of the ionization current with the thickness of the chamber walls. Combining the two results, it is possible to predict theoretically the ionization produced in a chamber of given wall material and wall thickness by a given flux of radiation of a particular energy. The theoretical expression involves knowledge of the stopping power of the chamber material for electrons of all energies up to the maximum produced by the radiation. These stopping power values are not known as accurately as one would wish, there