A systematic approach is described for the fabrication of defect-free high-silica zeolite membranes with CHA (SSZ-13) topology. Home-made hydrothermally-synthesized CHA seeds were coated on porous a-alumina substrates with a pore diameter of 80 nm and by means of a further hydrothermal treatment a zeolite membrane layer was formed. In order to obtain a thin and defect-free zeolite layer, the influence on the final microstructure of seed concentration during coating, coating method (rubbing, dip-or spin-coating) and crystal growth time was investigated. The template removal procedure was optimized to avoid the formation of cracks or defects. For an optimal thermal treatment, using a step-wise temperature increase to 500 degrees C, the membranes exhibit CO2/CH4 permselectivities of 25-30 with CO2 permeances of around 2 x 10(-7) molm(-2) s(-1) Pa-1 at 22 degrees C and 2 bar of pressure difference. O-2 plasma pre-treatment prior to template removal increased the CO2/CH4 permselectivity to 176, while maintaining the same CO2 permeance values when no pre-treatment was used. The SF6 permeances, both at low (22 degrees C) and high (200 degrees C) temperatures, were below the detection limit (2 x 10(-10) molm(-2) s(-1) Pa-1), which in return results in very high N-2/SF6 permselectivities of more than 700.