[en] The BEATRIX-II irradiation experiment was an in situ tritium recovery experiment to evaluate the tritium release characteristics of fusion ceramic breeder materials and to characterize their stability under fast neutron irradiation to extended burnups. This is an International Energy Agency (IEA) sponsored experiment which is being carried out in the Materials Open Test Assembly of Fast Flux Test Facility (FFTF). The participants are Japan, Canada and the US The in situ tritium recovery experiment consisted of two individual in-reactor experimental assemblies (Phase I and Phase II) that were irradiated for 300 and 200 EFPD, respectively. Each experimental phase included two specimens: a thin annular specimen capable of temperature changes and a larger temperature-gradient specimen. In Phase I both specimens were Li₂O while for Phase II the temperature-change specimen consisted of Li₂O and the temperature-gradient specimen was a Li₂ZrO₃ spherebed. Real-time measurements of the tritium release from the specimens during changing conditions (neutronics, temperature and sweep gas composition) were made using ion chambers. In order to correctly interpret the response of the ionization chambers it is necessary to understand the effect of changing sweep gas composition on the operation of the chambers. The purpose of this paper is to describe activities carried out to determine the effect of hydrogen additions to a helium sweep gas on the operation of these ionization chambers