There is described the preparation of hydrophilic, semi-permeable film membranes having an increase in pores and containing complex-forming metals. The films can be formed from solutions having film-forming material and pore-forming material dissolved in a solvent. The films can be useful for separating a component, e.g. an aliphatically-unsaturated hydrocarbon, from mixtures by the combined use of liquid barrier permeation and metal complexing techniques. The liquid barrier is at least partially within the hydrophilic film membrane during use, and the barrier contains complex-forming metal ions in aqueous solution. The metal ions may be, for example, noble metal, nickel, mercurous, cuprous or other metal ions, and mixtures of these metal ions, and the aqueous solution may contain other cations. The separation of ethylene from ethane and methane is of particular interest.