A synthetic mixture of solutes (colloids, macromolecules, small organic molecules, and salts) was evaluated to facilitate consistent bench and small pilot-scale studies of membrane flux decline. In this paper, we present physical, chemical, and filtration resistance properties of inexpensive, commercial, off-the-shelf components -colloidal silica and dried whey protein in pH-buffered aqueous electrolyte solution - that we used to investigate membrane flux decline with ultrafiltration membranes. We also include some comparisons of their properties with those reported in the literature for natural waters. Our data shows that these synthetic components have many comparable physical and chemical properties as the constituents of natural waters. These results suggest that a well-characterized "surrogate" mixture for natural waters can be used for ongoing studies of membrane and membrane process performance and remove ambiguities that can result from seasonal, geographical, and storage-related variations in natural waters. Thus, the use of a surrogate mixture facilitates equitable comparisons among materials, equipment, and processes which would assure better-informed engineering decisions. (c) 2005 Elsevier B.V. All rights reserved.