The present invention is for novel high performance cross-linkable and cross-linked mixed matrix membranes and the use of such membranes for separations such as for CO2/CH4, H2/CH4 and propylene/propane separations. More specifically, the invention involves the preparation of cross-linkable and cross-linked mixed matrix membranes (MMMs). The cross-linkable MMMs were prepared by incorporating microporous molecular sieves or soluble high surface area microporous polymers (PIMs) as dispersed microporous fillers into a continuous cross-linkable polymer matrix. The cross-linked MMMs were prepared by UV-cross-linking the cross-linkable MMMs containing cross-linkable polymer matrix such as BP-55 polyimide. Pure gas permeation test results demonstrated that both types of MMMs exhibited higher performance for CO2/CH4 and H2/CH4 separations than those of the corresponding cross-linkable and cross-linked pure polymer matrices.