We have examined the properties of a new class of microfiltration and ultrafiltration membranes that are fabricated by assembling particles onto the surface of a microporous substrate and stabilizing the porous array into a composite. The particle array contains interstitial voids having a narrow size distribution that serve as channels for size sieving. This aqueous based technology has advantages relative to other membrane fabrication methods in terms of the control of asymmetry, the facile adjustment of pore size, and the ability to easily modify pore surfaces during the synthesis of particles. In this work we study the properties of the membranes (gas and water permeabilities) fabricated from different size particles and of varying thickness on a number of different supports. The experimental data is then analyzed with a standard model, Carman-Kozeny, to develop guidelines for the design of such membranes. For all of the composites, the volume porosity was found to be approximately 0.3, close to what would be expected for hexagonal closest packed array which corresponds to the visual appearance from electron micrographs. In this study, membranes with narrow pore size distributions from 0.038 to 0.122 mum were fabricated with fluxes three to four times higher than the commercial membranes of similar pore size manufactured by phase inversion processes. (C) 2003 Elsevier B.V. All rights reserved.