NOVELTY - A fluid separation membrane contains polyimide polymer and blend polymer chosen from polyamide polymer and/or polyamide-imide polymer. The polyimide polymer contains several repeating units (A-C) (I-III).    USE - For fluid separation (claimed) used for production of oxygen-enriched air and nitrogen-enriched air for inerting and blanketing, for separation of carbon dioxide from hydrocarbons, and separation of hydrogen from petrochemical and oil refining streams. Also used for removing carbon dioxide and hydrogen sulfide from raw natural gas to form natural gas of pipeline quality.    ADVANTAGE - The fluid separation membrane exhibits resistance to interaction with the process components or contaminants, which leads to plasticization of membrane. The membrane has favorable mechanical properties, which results in excellent performance when exposed to high membrane differential pressure and high process temperature. The membrane has good balance of high productivity and selectivity for fluids and has favorable separation performance even when contact with aggressive process composition, pressure and temperature conditions for long period of time.    DETAILED DESCRIPTION - A fluid separation membrane contains polyimide polymer and blend polymer chosen from polyamide polymer and/or polyamide-imide polymer. The polyimide polymer contains several repeating units (A-C). The repeating unit (A) is of formula (I). The repeating unit (B) is of formula (II): -(C(=O)-R2-C(=O)-NH-Ra-NH)-. The repeating unit (C) is chosen from moieties of formulae (I-III).    R1=moiety chosen from formulae (A,B and/or C);    R4=moiety chosen from formulae (Q,S and/or T);    Z=moiety chosen from groups such as -C(=O)-, -O-, and/or -SO2-;    Ra=moiety chosen from formulae (a-g);    Z'=moiety chosen from formulae (h, j-l);    X,X1,X2,X3=moieties chosen from hydrogen and 1-6C alkyl;    Z"=-O- or -C(X)(X)-;    R2=moiety chosen from formulae (q,s);and    R3=group of formula (t).    An INDEPENDENT CLAIM is included for separation of one or more fluids from a fluid mixture. A fluid mixture is contacted with one side (A) of fluid separation membrane such that a preferentially permeable fluid of fluid mixture is permeated through the separation membrane faster than the less preferentially permeable fluid, to form a permeate fluid mixture enriched in preferentially permeable fluid on other side (B) of fluid separation membrane, and a retentate fluid mixture depleted in preferentially permeable fluid on side (A) of fluid separation membrane. Thus, separation of one or more fluids from fluid mixture is carried out.