All-silica CHA nanocrystals, much smaller (20–200 nm) than previously reported, were prepared by an improved method developed in the present work. The nanocrystals are prepared by adding milled crystals to a fluoride synthesis mixture and we observed that much smaller crystals are obtained by adding a much higher fraction of milled crystals. In the next step, CHA membranes with a thickness of ca. 1.3 μm were prepared by hydrothermal treatment of a monolayer of nanocrystals supported on porous graded alumina discs in a fluoride synthesis gel. Finally, the membranes were calcined at 480 °C. The highest measured single gas CO₂ permeance was 172 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at room temperature. The highly permeable membranes were evaluated for separation of CO₂ from an equimolar mixture with CH₄ at varying temperatures. The highest observed CO₂ mixture permeance was 84 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 276 K with a separation selectivity of 47 at 9 bar feed pressure and atmospheric permeate pressure. At room temperature, the CO₂ mixture permeance was also as high as 78 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ with a separation selectivity of 32. To the best of our knowledge, these CO₂ permeances are by far the highest reported for CHA membranes, while the selectivity is similar to that reported previously at comparable test conditions.