The structure of membrane cakes formed in cross-flow filtration was investigated for conditions of variable particle size and solution ionic strength. In all cases, microscopic examination of membrane cake cross-sections revealed a stratified structure: a dense region of the colloidal deposit adjacent to the membrane with an abrupt transition to a much more porous layer near the membrane-suspension interface. The thickness and structure of individual layers varied as functions of particle size and solution ionic strength. Relative specific hydraulic resistances of individual layers were estimated from regressed transport coefficients in a model describing macroscopic transport and permeate flux. This work provides direct experimental confirmation of trends predicted from theory that couples hydrodynamic conditions and interparticle interactions to describe the permeate flux performance of cross-flow filtration membranes. (C) 2004 Elsevier B.V. All rights reserved.