Zeolite membranes with higher Si contents displayed higher resistances to water vapor in gas mixtures. CHA zeolite membranes from an all-silica gel/solution (namely, Si-CHA herein) were always prepared by the fluoride route. High-quality Si-CHA zeolite membranes were prepared using a green and fluorite-free synthesis route in a diluted all-silica solution via a single hydrothermal synthesis step in this study. The as-synthesized membranes had a compact CHA zeolite layer with a Si/Al ratio in the framework of similar to 75 and a thickness of 2.5 mu m. A trace of aluminum in the membrane came from the alumina support. Separation performances through Si-CHA zeolite membranes as functions of pressure drop and temperature were measured. The best membrane displayed a CO2 permeance of 1.2 x 10(-6) mol (m(2) s Pa)(-1) and CO2/CH4 selectivity as high as 480 for an equimolar CO2/CH4 gas mixture at 298 K and 0.2 MPa pressure drop. Si-CHA zeolite membranes displayed good and stable separation performance in a wet mixture.