TECHNOLOGY FOCUS - POLYMERS - Preferred Component: The continuous phase polymer is optionally substituted polymers from polysulfones; poly(styrenes) including styrene-containing copolymers such as acrylonitrilestyrene copolymers, styrene-butadiene copolymers, or styrene-vinylbenzylhalide copolymers; polycarbonates; cellulosic polymers, such as cellulose acetate-butyrate, cellulose propionate, ethyl cellulose, methyl cellulose, or nitrocellulose; polyamides and polyimides, including aryl polyamides and aryl polyimides; polyethers; polyetherimides; polyetherketones; poly(arylene oxides) such as poly(phenylene oxide) and poly(xylene oxide); poly(esteramide-diisocyanate); polyurethanes; polyesters (including polyarylates), such as poly(ethylene terephthalate), poly(alkyl methacrylates), poly(acrylates), or poly(phenylene terephthalate); polypyrrolones; polysulfides; polymers from monomers having alpha-olefinic unsaturation other than mentioned above such as poly(ethylene), poly(propylene), poly(butene-1), poly(4-methyl pentene-1), polyvinyls, e.g., poly(vinyl chloride), poly(vinyl fluoride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinyl alcohol), poly(vinyl esters) such as poly(vinyl acetate) and poly(vinyl propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes) such as poly(vinyl formal) and poly(vinyl butyral), poly(vinyl amides), poly(vinyl amines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinyl phosphates), and poly(vinyl sulfates); polyallyls; poly(benzobenzimidazole); polyhydrazides; polyoxadiazoles; polytriazoles; poly(benzimidazole); polycarbodiimides; polyphosphazines; and interpolymers, including block interpolymers containing repeating units from the above such as terpolymers of acrylonitride-vinyl bromide-sodium salt of para-sulfophenylmethallyl ethers; and grafts and blends containing typical substituents providing substituted polymers including halogens such as fluorine, chlorine and bromine; hydroxyl groups; lower alkyl groups; lower alkoxy groups; monocyclic aryl; or lower acyl groups. The membrane is in the form of hollow fiber. At least one of the polymer which is pyrolyzed and the polymer comprising the continuous phase polymer is Ultem 1000 (RTM), Matramid 5218 (RTM), 6FDA/BPDA-DAM, 6FDA-6FpDA, or 6FDA-IPDA.Preferred Composition: The volume ratio of particles to continuous phase polymer is 0.1-1.5, preferably 0.25-1.Preferred Property: The membrane is 1000 Angstrom and 0.005 inch in thickness. The sieve particles are fragments of a pyrolyzed polymer powder and contain ultramicropores produced by fracturing of the pyrolyzed powder to break open closed-ended pores. The size of the sieve particles is less than 1, preferably 0.9 mum. The discrete sieve particles have pores which are in the size of 3-10, preferably 3-5 Angstrom. The continuous phase polymer is a rigid, glassy polymer having a glassy transition temperature of greater than 150 degreesC.Preferred Method: The method also includes priming the sieve particles with a polymer including dispersing the sieve particles in a solvent, adding a small amount of a polymer to the dispersion, where the polymer is soluble in the solvent, and adding a non-solvent for the polymer to the dispersion to initiate precipitation of the polymer into the sieve particles. The step of processing the pyrolyzed polymer includes fracturing the pyrolyzed polymer to form sieve particles of a desired size. After the mixed membrane is formed, the membrane is subjected to an annealing treatment by heating the membrane above the glass transition temperature of the polymer.Preferred Membrane: The mixed matrix membrane includes discrete sieve particles containing pores of an effective size such that when the particles are dispersed in the continuous phase polymer and a mixed matrix membrane is prepared having 20 wt.% loading of particles, the membrane exhibits a pure gas selectivity enhancement of at least 10% in permeability of CO2 relative to CH4 when compared against a membrane made of the same continuous phase polymer but without the discrete sieve particles. EXAMPLE - Carbon molecular sieve (CMS) particles (0.2 g) were added with dichloromethane CH2Cl2 (3 ml), and were sonicated in solution for 1 minute. The slurry was agitated and mixed for 2 hours. The large CMS particles in the slurry were separated from the fine particles by decantation or centrifugation. The slurry was taken and allowed to stand for 6 hours to settle large particles. The finer particles (0.184 g) dispersed in solution were decanted. Ultem 1000 (RTM)(0.025 g) was added to the solution slurry and mixed for 12 hours. Ultem 1000 (RTM)(0.713 g) was added to the solution to form a solution with 20 wt.% or 17.5 vol.% of CMS particles. The solution was mixed for 12 hours. The polymer/CMS solution was poured into a glass surface placed inside a controlled environment, and spread to a uniform thickness. The solvent was slowly evaporated for 12 hours. The film was 40 mum in size, and was annealed at 100 degreesC for 12 hours in vacuum.