Silicalite-1 membranes were prepared on mullite supports in a fluoride containing solution by a secondary growth method. Influence of seed crystal size was examined by applying seeds of 100-200 nm and 1-2 mu m, respectively. Randomly oriented poly-crystalline MFI membranes were obtained with larger seeds. On the contrary, membranes formed by applying smaller seeds became more c-axis oriented with longer synthesis time. While seed size had no significant influence on hydrogen permeance, SF6 permeance was ca. three times smaller when smaller seeds were used. Applying smaller seeds and fluoride containing synthesis solution, MFI membranes having H-2/SF6 ideal selectivity over 100 were obtained within 6 h.