Carbon dioxide separation has drawn much interest because of the concern over global warming. A simple and yet effective way of removing CO2 is highly desirable for the separation involving combustion effluents, consisting mainly of CO2 and N-2, hydrogen production involving CO2 and H-2 mixtures, and natural gas sweetening involving mainly CO2 and CH4. Here, we present water membrane for CO2 separation. The water membrane is simply a water layer formed on a hydrophobic support membrane to prevent the water from permeating through the support layer. The membrane exploits the fact that the solubility of CO2 in water is almost two orders of magnitude higher than the solubility of the gases of N-2, H-2, and CH4. Both the permeance and the selectivity increases with decreasing water layer thickness, contrasting the traditional tradeoff between permeance and selectivity. The carbon dioxide selectivity with respect to nitrogen, methane, and hydrogen is 86, 66, and 74, respectively. The permeance of 1.1 GPU reached with 5 mm thick water layer should improve in due time with further work on reducing the thickness.