In any real separation application, membranes are exposed not to pure gases but to gas mixtures. Separation performance is usually estimated by comparing permeabilities of gases tested individually, but many gases interact with the membrane and with each other in ways that influence each other’s permeability. Materials that perform well in pure gas laboratory tests often fail to perform under real world conditions, but testing under these conditions requires sophisticated equipment. To meet this need, the AMPMC created ten permeation boxes that can test gas mixtures in addition to single gases, at pressures up to 50 bar, using a variation of the standard time lag method. Mixture inlet and outlet compositions are determined by gas chromatograph using a sampling technique developed at KAUST.