Several commercially available organic materials have frequently been used as surrogates for surface water natural organic matter (NOM) and for wastewater effluent organic matter (EfOM), e.g., for identifying the extent and mechanism of membrane fouling. However there is a lack of experimental evidence that the surrogate organic materials are representative of EfOM and surface water NOM. Therefore the main objectives were to compare the organic composition (by empirical fractionation methods), the molecular size distributions, and fouling and the mechanisms of fouling of ultrafiltration (UF) membranes for the surrogate materials sodium alginate, soluble microbial products (SMP), and commercial humic acid (HA) with the characteristics of EfOM and Suwannee River NOM (SRNOM). The fouling mechanisms were probed by measuring resistance to filtration as a function of permeate flux using several filtration modes, by determining the effects of Ca2+ on fouling, and by investigating the effectiveness of hydraulic and chemical cleaning on restoration of membrane permeability. Results showed that SMP was most similar to EfOM, Aldrich HA (AHA) was most similar to SRNOM, and alginate was a poor surrogate for either EfOM or SRNOM. Alginate was also unique among the five organic materials in that fouling was decreased when 0.8 mM Ca2+ was added, consistent with the formation of a gel that formed a porous cake on top of the membrane. Fouling increased with addition of Ca2+ for the other materials. Addition of Ca2+ increased the colloidal fraction and the concentration of molecules >100 kDa for all of the materials but the increases were particularly dramatic for alginate and were also large for AHA. Differences in fouling mechanisms and cake compressibility as a function of the applied trans-membrane pressure (TMP) between surrogate and natural organic materials were also observed. (C) 2012 Elsevier B.V. All rights reserved.