Multi-block co-polyimides 6FDA-CARDO/6FDA-Durene with varying segmental lengths were synthesized and pure and mixed sour gas transport properties through dense membranes were investigated as function of feed pressure, temperature and gas composition. Pure gas measurement indicated that block co-polyimide 6FDA-CARDO/6FDA-Durene (2500/2500) membrane exhibits high permeation properties and separation characteristics with CO2 permeability and CO2/CH4 selectivity of up to 239 barrer and 28 respectively. The block co-polyimide also showed excellent performance under harsh sour gas environment (i.e. high H2S content of up to 36 vol% and feed pressure of up to 55 bar for a gas mixture consisting of CO2, CH4, N-2, C2H6 and H2S). At 36 vol % H2S, the membrane exhibits H2S permeability and H2S/CH4 selectivity of up to 275 barrer and 24 respectively. These values and separation performance exhibited by the co-polyimide are comparable and very competitive even, as compared to the values obtained in some of the high performance polymeric membranes that have been reported in the literature. The stability of the block co-polyimide 6FDA-CARDO/6FDA-Durene membrane under these aggressive environments is quite impressive.