The paper describes the preparation and investigation of membranes based on new interpenetrating polymer network (IPN) of vinyl terminated poly(dimethyl siloxane) (PDMS) and aromatic polyimide (PI) with respect to their thermal and pervaporation properties. The modified membranes were prepared using simultaneous IPN (SIPN) technique by variation of polyimide loading of 5,10 and 15 wt% respectively. These membranes were characterized by different thermal, mechanical, morphological, spectroscopic and pervaporative techniques and compared with those of neat PDMS membranes. The IPN membranes exhibited synergistic improvement in the thermal stability in the range of 445-490 degrees C in air and 410-520 degrees C in inert atmosphere for 10% loss. Activation energies for the decomposition of polymers and their IPN's were calculated using Coats and Redfern equation. Permeation properties of PDMS and IPN blends were evaluated by water diffusion, measured by Fourier transform-attenuated total reflectance (FT-ATR) method and moisture vapour transmission rate (MVTR) as per ASTM E 96. 15 wt% polyimide content in PDMS membrane slows down the water diffusion and MVTR significantly. All the IPN's form mechanically strong membrane with tensile strength up to 15.5 MPa and elongation at break up to 20%. IPN membranes prepared in this work were employed in pervaporation separation of azeotrope forming toluene/methanol mixtures. The pervaporation properties could be tuned by adjusting the blend composition. All the blend membranes tested showed a decrease in flux with increasing polyimide content for methanol/toluene liquid mixtures. Toluene permeated preferentially through all tested blend membranes, and the selectivity increased with increasing polyimide content. The pervaporation characteristics of the blend membranes were also strongly influenced by the feed mixture composition. The flux increased exponentially with increasing toluene concentration in the feed mixtures, whereas the selectivities decreased for liquid mixtures. This study demonstrates that polymer IPN blends is a simple way to modulate membrane's transport properties and can achieve higher performance than the pristine polymer materials. (c) 2010 Elsevier B.V. All rights reserved.