A new, highly selective silicon-based membrane was developed by pyrolyzing a thermosetting silicon resin precursor in two steps. The pyrolysis step was performed under an inert gas in the range of 800 to 950 degrees C, The activation step was conducted at the same temperature as the first step, Oxygen content in the activation gas was located between 0.5 and 1.0%, The crosslinking density and the matrix structure of the polymeric precursor were important factors for the permeable characteristics of the resulting pyrolysis membrane, The average selectivity was 5 to 10 times those of a pyrolysis membrane from silicon rubber, Additives in the precursor had significant effects on toughness and stability of the resulting membrane. By varying the additive content in the precursor, silicon-based pyrolysis membranes with different stabilities in air were obtained.