(JP7251050-A)      A blend comprises: (a) an aromatic polyethersulphone; (b) an aromatic polyimide; and (c) an alkyl-substd. aromatic polyimide, polyamide and/or polyamide-imide. The aromatic polyethersulphone contains at least one unit of formula (I); Q = nothing or is of formula (Ia) or (Ib); t =0 or 1; Q' = C(CF3)2, -C(Z)2-, -S(=O)2-, -Si(Z)2, -Si(Z)2-O-Si(Z)2- and/or -C(+O)-; Z = H, 1-6C alkyl gp. 6-12C aromatic gp. or F, Cl, Br or I (pref. H); and m = 0-4. The aromatic polyimide contains at least one unit of formula (II); Ar" = a gp. of formula (II); Ar" = a gp. of formula (IIa); Y = 1-6C alkyl gp. or 6-12C aromatic gp. and R = as defined below. The alkyl-substd. aromatic polyimide contains at least one unit of formula (III), Ar = aromatic diamine moiety of formula (IIIa) or (IIIb); Ar' = a gp. of formula (IIIc); X = nothing or R'; R' = a gp. of formula (IIId) or (IIIe), -C(CF3)2-, -C(Z)2-, -S(+O)2-, -Si(CH3)2-, -Si(Z)2-, -Si(Z)2-O-Si(Z)2 and/or -C(+O)-; Z = H, 1-6C alkyl gp. -12C aromatic gp. F, Cl, Br or I; n = 0-4; R'' = -C(CF3)2-, -C(Z)2-, -S(+O)2-, -Si(CH3)2-, -Si(Z)2-, -Si(Z)2-O-Si(Z)2-, -C(+O)- and/or a gp. of formula (IIIf); R = gp. of formula (IIIg), (IIIh), (IIIi) and/or (IIIj); R''' = as R'. The alkyl-substd. aromatic polyamide contains at least one unit of formula (IV) Ar = as above; and Ra = aromatic diacid moiety. The alkyl substd. aromatic polyamide-imide contains at least one unit of formula (V); Ar, R and Ra = as above; Rb = aromatic moiety; and x, y, and z = fractions where x+y+z = 1. Alternatively, the alkyl-substd. aromatic polyamide-imide may be a mixt. of the alkyl-substd. aromatic polyimide and alkyl-substd. aromatic polyamide described above.    Also claimed are; (i) a membranous structure for gas sepn. comprising a miscible blend of (a) and (b) as above; and (ii) a process for separating one or more gases from a gaseous mixt. which comprises bringing the mixt. into contact with a first side of a membrane structure as above, in a manner to cause a portion of the mixt. to pass through the membrane to permeate a side, the resulting gas mixt. on the permeate side being enriched in one or more component over that of the mixt. on the first side.    USE -   The use (claimed) of the compsn. is to mfr. sepn. membranes, partic. gas sepn. membranes, e.g. for sepg. H2 from other gases including N2, CO and CH4, for sepg. CO2, H2S and water vapour from natural gas and the enrichment of air by nitrogen or oxygen. In addn. hydrogen can be recovered from ammonia prodn. plants and coal gasification processes.    ADVANTAGE -   The gas sepn. membranes can be cast from a two-component or three-component blend of the polymers. The polymers can be mixed in any ratio to achieve the desired properties of the membrane. The membranes achieve improved gas transport properties and the productivity and selectivity may be conveniently engineered to achieve desired final properties.