PURPOSE: To obtain a composite membrane consisting of a thin active layer of polyurea/urethane by coating a microporous support substrate with a polyurea/urethane copolymer soln. consisting of an aprotic solvent, a cyclic ether, Cellosolve acetate or methyl Cellosolve and a wetting agent.

CONSTITUTION: A mixture of about 3-27% aprotic solvent such as dimethylformamide or the like, about 94-33% cyclic ether such as a dioxane or the like, about 2-33% Cellosolve acetate or methyl Cellosolve and about 1-7% wetting agent such as crotyl alcohol or the like is added to 100 pts. solvent in an amt. of about 40 pts. to prepare a polymer/solvent soln. This polymer/ solvent soln. is applied to a microporous support substrate to deposit a thin active layer of polyurea/urethane or polyurea. The thin film composite film thus formed is useful for separating an aromatic hydrocarbon from a saturated hydrocarbon.