Membrane and thermal desalination are plagued by composite fouling because of inorganic, biological and organic constituents. Interactive effects in composite fouling, though of paramount practical importance, are mostly ignored. This paper presents interactive effects of biological matter, Pseudomonas Fluorescence (PF), on precipitation of calcium sulfate, a sparingly soluble salt and an inorganic foulant. Experiments were conducted in isothermal (25 degrees C) batch tests. The effect of PF and its concentration on CaSO4 precipitation and deposit structure at varying salinities were determined. The Pitzer model, suitable for high salinities, is used in calculation of instantaneous CaSO4's ion activity products. The Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) show that salinity and concentration levels alter the crystal structure but not the crystal phase (gypsum). Under the influence of PF, crystallization of CaSO4 did not occur and sulfur levels significantly increased; it can be surmised that microbial activity of PF reduced the sulfate ions to elemental sulfur and altered precipitation of CaSO4. This does not suggest that biological fouling or presence of bacteria generally inhibits precipitation fouling of calcium sulfate; it underlines that the generally ignored interactive effects are of significant importance and should be considered as corner stone in advancement of fouling research. (C) 2011 Elsevier B.V. All rights reserved.