NOVELTY - Preparing a silicone membrane involves: contacting at least one surface of a film comprising a silicone elastomer having a set of silicon-bonded hydrogen atoms with a composition comprising an unsaturated poly(alkylene oxide), and optionally, a platinum group metal-containing catalyst, for an amount of time sufficient to convert at least a portion of the silicon-bonded hydrogen atoms to silicon-bonded carbon groups, giving a silicone membrane having a carbon dioxide (CO2) permeance of at least about 8-30 GPU.    USE - The method is useful for preparing a silicone membrane used in separating gas components in a feed gas mixture (claimed).    ADVANTAGE - The method efficiently prepares a silicone membrane exhibiting both high permeability and selectivity for particular components in a gas mixture.    DETAILED DESCRIPTION - Preparing a silicone membrane involves: contacting at least one surface of a film comprising a silicone elastomer having a set of silicon-bonded hydrogen atoms with a composition comprising an unsaturated poly(alkylene oxide) having the formula R1O(R2O)nR3 and, optionally, a platinum group metal-containing catalyst, for an amount of time sufficient to convert at least a portion of the silicon-bonded hydrogen atoms to silicon-bonded carbon groups, giving a silicone membrane having a carbon dioxide (CO2) permeance of at least about 8-30 GPU.    R1=an organic group having at least one unsaturated aliphatic carbon-carbon bond;    R2=2-4C hydrocarbylene;    R3=R1, H, alkyl, aryl, acyl, alkylacyl, alkoxyacyl, or an epoxy-functional group; and    n=2-30.    INDEPENDENT CLAIMS are included for:    (1) an unsupported silicone membrane prepared by the method, selected from a plate membrane, a spiral membrane, tubular membrane, hollow fiber membrane, and a combination;    (2) a method of separating gas components in a feed gas mixture, which involves contacting a first side of the silicone membrane with a feed gas mixture comprising at least a first gas component and a second gas component to produce a permeate gas mixture on a second side of the membrane and a retentate gas mixture on the first side of the membrane, where the permeate gas mixture is enriched in the first gas component, and the retentate gas mixture is depleted in the first gas component; and    (3) a silicone membrane, which comprises: a cured product of a composition-contacted film, the composition-contacted film comprising a film comprising a silicone elastomer having a set of silicon-bonded hydrogen atoms; and a composition in contact with at least one side of the film, comprising the unsaturated poly(alkylene oxide).