Because of the impact of poor water clarity on defueling operations at the Three Mile Island Unit 2 nuclear power station, a study was undertaken to characterize suspended particulates in the reactor coolant system water. Three defueling water samples taken at different times after varying degrees of clarification were filtered through a series of Nuclepore filters of decreasing pore sizes, and the collected solids were examined with a scanning electron microscope and analyzed by X-ray fluorescence. Limited tests were also made using polyelectrolyte and diatomaceous earth (DE) additives then under study for improving filtration properties. Samples of defueling water cleanup system (DWCS) filter media, which had been tested with similar reactor water and had quickly plugged, were also examined using several techniques. A wide variety of solids was observed on the Nuclepore filters, and analytical data confirmed the presence of at least 26 elements. These included all the major constituents from the reactor system, chemicals and zeolites used to decontaminate the water, and common impurities, as well as a few unexpected elements. There was also evidence for organic solids. Thus, a diverse assortment of particles varying in size from >10 to <0.1 µm was present. The filters collected small particles that partially blocked the pore openings. In addition, the stainless steel DWCS test filters were coated with a thin surface film that changed in nature from predominantly organic for unclarified water to siliceous after water clarity had been markedly improved by filtration using DEs.