By employing fluids of the same composition (regarding foulants and ionic strength) specific fouling resistances are measured in the constant flux filtration mode, in dead-end UF and cross-flow RO tests, and compared with the respective resistances determined under constant pressure. Sodium alginate (SA) alone and mixtures with humic acids (HA), in presence of calcium, were used as typical organic foulants in feedwaters of two ionic strengths (TDS = 500 and 2000 mg/L). Plotting resistances a, from UF (or RO) constant pressure and constant flux tests, versus pressure drop across the cake Delta P-c, estimated at the same deposited mass density, very good agreement was obtained; increasing resistance alpha with Delta P-c was observed due to cake compressibility, as expected. Regarding cake structure, the dominant role of alginates, in proportion greater than 25% in mixtures with HA, is reflected in the resistances, which exhibit approximately the same compressibility index. Moreover, at the higher alginate proportions and for the greater salinity (TDS 2000 mg/L), the resistances from both UF and RO tests are correlated satisfactorily, versus Delta P-c, by a single expression. These results significantly contribute towards developing reliable tools for assessing water fouling potential and predicting RO membrane fouling evolution in desalination plants. (C) 2012 Elsevier B.V. All rights reserved.