Recent NASA-funded studies of Allied-Signal metal-oxide-based absorbents demonstrated that these absorbents offer a unique capability to remove both metabolic carbon dioxide (CO2) and water (H2O) vapor from breathing air; previously, metal oxides were considered only for the removal of CO2. The concurrent removal of CO2 and H2O vapor can simplify the astronaut portable life support system (PLSS) by combining the CO2 and humidity control functions into one component. A further benefit is that the removal processes are reversible, permitting a regenerative component. Thus, a metal oxide absorbent offers many advantages over the current system, which is nonregenerative and uses separate processes for CO2 and H2O vapor removal. These advantages include lower complexity, lower maintenance, and longer life.The use of metal oxide absorbents for removal of both CO2 and H2O vapor in the PLSS is the focus of an ongoing NASA program. Recently, two metal oxide formulations previously shown to have promising CO2 and H2O vapor uptake characteristics, durability, and long-term regenerative performance were evaluated in a subscale unit operationally representative of full scale. A mixed metal oxide formulation was found to be superior to silver oxide and was selected for further evaluation in a full-scale closed-loop testing scheduled to begin in August 1991. This paper presents the test results that provide the design basis for a full-scale preprototype unit. This metal oxide system effectively performs the functions of both CO2 and H2O vapor removal and will significantly reduce the complexity and maintainability of a PLSS while allowing long-term reuse.