In situ hydrothermal synthesis of silicalite layers on rnacroporous alpha-Al2O3 disks were performed from a batch composition of 6.5Na(2)O:25SiO(2):xH(2)O:6.9TPABr by varying the water content of a clear crystallization solution from 500 to 2000 moles at 200 degreesC. As the water content of the clear solution was increased, continuous and thinner silicalite layers formed from smaller crystals were obtained and the layer quality was improved. The silicalite layer which was synthesized from the most concentrated batch was formed from crystals with an average size of 100 mum and had a thickness of 36 mum. The layer thickness and average crystal size decreased to about 8 and 7 mum, respectively, when the batch containing 1400 moles of water was used for synthesis. The results showed that the layer thickness on porous alumina supports could be adjusted by varying the water content of the synthesis solution. As-synthesized membranes were impermeable to N-2 after a single crystallization step. The membranes were tested for possible pinholes and microcracks by measuring single gas permeations of N-2 and SF6 after calcination at 500 degreesC by a slow and step-wise calcination procedure. The calcined membranes showed an N-2 permeance of 16.3 mmoV(bar m(2) s) and an ideal selectivity of 1630 for N-2/SF6 at room temperature. The high selectivity showed, that the silicalite layer is free of large interzeolitic pores and the applied calcination procedure is, suitable for TPA removal without forming microcracks. (C) 2002 Elsevier Science B.V. All rights reserved.