Much effort has been paid towards the preparation of hydrophilic zeolite LTA membranes, and these zeolite LTA membranes have showed excellent performance in hydrophilic separations. However, there are only a few reports on successful shape-selective separations of gas mixtures on zeolite LTA membranes, most separation factors reported so far were lower than the corresponding Knudsen constants. It is often found that zeolite LTA membranes contain inter-crystalline defects to degrade their separation selectivity, which result from the problems that the isolated crystals on the support surface grow together to form a continuous supported LTA layer. In the present work, via urethane bonds formed by the reaction of isocyanate groups with surface hydroxyls, a facile synthesis method is developed for the seeding-free preparation of supported dense zeolite LTA layers by using 1,4-diisocyanate as a molecular binder to anchor the zeolite nutrients onto the support surface. The α-Al2O3 supported zeolite LTA membrane with a thickness of about 3.5 μm displays molecular sieving performance in gas permeation tests. © 2011 The Royal Society of Chemistry.