Silane coated on magnesia particulates in isopropyl alcohol (IPA) medium, precluded the formation of brucite phase, which hitherto existed in silane coated on magnesia filler in aqueous medium. Both the types of coated MgO powders were separately filled in the FIFE matrix, and microwave flexible substrates were made by the process comprising of sigma mixing, extrusion, calendering and hot pressing (SMECH process). Filler content in the PTFE matrix was varied from 10 to 70 wt% and microstructures of the composites were analyzed using SEM technique. The dielectric properties of the planar substrates were measured at X-band using waveguide cavity perturbation technique. The IPC-TM-650 procedure was employed to evaluate the moisture absorption characteristics. (C) 2010 Elsevier B.V. All rights reserved.