Gas-permeable membranes that continuously transfer carbon dioxide (CO2) from air to water were investigated in an effort to bypass the operational limitations of expendable solid absorbents currently used for CO2 control in closed-circuit underwater breathing apparatus (UBA). Rebreather UBA CO2 control requirements and known membrane properties were used to create a functional hierarchy of membrane types and CO2 transfer mechanisms, from which one membrane configuration was selected for evaluation. This Direct Contact Membrane Separator (DCMS) employs microporous hydrophobic Hollow Fiber Membrane (HFM) modules to create large membrane areas in small volumes for air-water phase contact without intermixing. Since the micropores in the hydrophobic walls of the hollow fibers are air-filled, gas permeation rates through this membrane are far higher than for any solid or liquid membrane. Such HFMs, commonly used for degassing ultrapure water, are commercially available in a wide range of sizes. Subscale laboratory testing demonstrated the suitability of the DCMS for rebreather UBA CO2 control. The best CO2 transfer performance occurred with two-pass turbulent crossflow of water around the outside of the fibers, countercurrent to laminar air flow through the fibers. CO2 removal capacity was primarily dependent on the water-to-air flow rate ratio through the HFM and the CO2 content limit in DCMS outlet air; allowing DCMS outlet air CO2 content to rise from 0.5 vol% to 1.0 vol% would halve required membrane area and HFM volume. Mixed-gas rebreather UBA oxygen and inert diluent gas loss through this nonselective porous membrane to seawater was projected to have negligible impact on DCMS volume penalty, if UBA gas storage pressure is raised from the current 3000 psi to 5000 psi. Unmanned and manned laboratory and underwater testing of the DCMS (with a single full-scale HFM module) showed that this “artificial gill” is ready for manned submersible and diver application development.