The composition of Gas Mixtures can be influenced with the aid of microporous membranes by exploitation of the Knudsen effect. The components of lower molecular mass are enriched in the permeated gases while the components of higher molecular mass are concentrated in the retained gases. The best separation is achieved at higher Knudsen numbers. The maximum achievable separation factor is limited by the molecular mass ratio. Hence the gas components in the permeate can be enriched or depleted but never completely separated. In the present experiments, separation factors up to 1.39 and enrichment factors up to 1.10 were attained. However, these values should be subtantially improved on use of surface modified porous membranes. Use Of ceramic membrane materials permits use of this separation mechanism at very high temperatures.