Abstract: A process is described for the manufacture of crystalline molecular sieve layers with good para-xylene over meta-xylene selectivity's good para-xylene permeances and selectivities. The process requires impregnation of the support prior to hydrothermal synthesis using the seeded method and may be undertaken with pre-impregnation masking. The crystalline molecular sieve layer has a selectivity (&agr;x) for para-xylene over meta-xylene of 2 or greater and a permeance (Qx) for para-xylene of 3.27×10−8 mole(px)/m2.s.Pa(px) or greater measured at a temperature of ≧250° C. and an aromatic hydrocarbon partial pressure of ≧10×103 Pa.