The cosorption of water and carbon dioxide by molecular sieves is a potential method of removing these contaminants from the helium coolant of a nuclear gas cooled reactor. This system was experimentally investigated by both differential- and deep-bed tests at a temperature of 25°C; at pressures of 1 to 30 atm for differential tests and 10 to 30 atm for deep-bed tests; with gas flow rates of 0.0010 to 0.0138 g/(cm2 sec); and with inlet water or carbon dioxide concentrations of 3.4 × 10−8 to 9.3 × 10−7 g moles/cm3. These tests showed that the system could be described by the rate limiting step of intracrystalline diffusion with diffusion coefficients at 25°C of 1.92 × 10−10 cm2/sec for water and 3.11 × 10−10 cm2/sec for CO2. Sorbed CO2 was found to be irreversibly replaced by sorbed water, and the CO2 loading was dependent on water concentration. Differential equations were derived to describe the system of the cosorption of two interacting fluid species with Freundlich-type isotherms in a flowing fluid by a fixed bed of solids in which the sorption rate is controlled by intracrystalline diffusion. The set of differential equations was solved by a finite difference method for the case of water and carbon dioxide cosorption by molecular sieves. Generalized breakthrough curves for both water and CO2 were determined, and their use for design purposes is demonstrated.