In this work, membranes based on polycyclic silicone networks were synthesized from different cyclic siloxanes, through hydrosilylation reactions. These membranes showed good thermal stability, glass transition higher than conventional silicone networks and the absence of pores and phase segregation, with roughness surface. The nanomorphology of the materials was studied by small angle X-ray scattering. Some samples produced practically no scattering and some produced patterns that are typical of non-particulate systems. The gas permeability behavior of the membranes was investigated, showing permeability coefficients higher than those observed for some organic polymers, and selectivity coefficients higher than those found for PDMS based membranes. (C) 2000 Elsevier Science Ltd. All rights reserved.42413191327