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As composite membranes are made with ever thinner top layers, the influence of the support membrane on composite membrane performance increases. This influence goes beyond the transport resistance of the support membrane becoming a rate limiting factor. We will show that the surface structure of the support membranes just by itself can reduce membrane permeance. The analysis shows that isoporous surfaces are uniquely suited to serve as support membranes. Experimental data will be presented plus the results of analytical methods that provide insights into the formation of isoporous surfaces.