Softening is designed to remove hardness ions, but it can also remove NOM and particles, yielding the possibility to use the process as a pretreatment for ultrafiltration. The objectives of this research were to understand the nature of the fouling mechanisms for ultrafiltration when used for waters that either require softening or have been softened, and to use that understanding to determine promising options for the use of softening as a pretreatment before ultrafiltration. To understand fouling mechanisms in the integrated system with softening and ultrafiltration, three different levels of softening performance in terms of removal of inorganics and organic matter were selected. Experiments were performed with both natural waters and synthetic waters with similar (but separable) inorganic, organic, and particulate characteristics. The synthetic waters were used to distinguish among inorganic fouling by precipitates, organic fouling, particulate fouling, and combined fouling by particles and organic matter. The results showed that organic matter played a major role in fouling, either by itself or by adsorption onto particles, and that softening pretreatment effectively reduced the foulants prior to ultrafiltration.