This study focuses on fouling while utilizing hollow fiber membranes as wetting media in evaporative cooling applications. A duct-mountable, hollow fiber membrane array is fed tap water or water with a known concentration of calcium sulfate or calcium carbonate to simulate evaporative cooling with scale forming waters. Mass transfer coefficients are experimentally derived from collected data, and concentration polarization effects and fouling mechanisms are quantified from changes in the measured mass and heat transfer coefficients along with visual verification from SEM imaging. Results indicate that membrane fouling can be characterized by a decrease in the effective porosity, the extent of fouling can be related to mineral solution concentrations within the membrane, and that a fouling induction period and wasting flow rates influence fouling rates. (C) 2008 Elsevier B.V. All rights reserved.