The corrosion products formed in boiling water reactor (BWR) coolant systems cause a wide range of problems due to reduced heat transfer and transportation of radioactivity. It is of prime interest to describe corrosion product properties in order to form a basis for the reduction of their negative influence on plant performance. Corrosion product particle characterization was carried out in connection with a study of precoat filtration of condensate water in eight Swedish and two Finnish BWRs. A variety of different techniques and tools were used in the characterization work. Filtration was used for the capture of particles, and scanning electron microscopy was used for size measurements, surface studies, and analysis of the elements present. The X-ray diffraction technique was usedfor phase determinations. A wide range of iron-containing particulate material is present in the water of different BWR systems. The corrosion products are strongly dominated by particulate material. Most particles are in the colloidal size range and are composed of small crystallites or amorphous material and normally have a negative surface charge. The largest number of particles in condensates is found in the submicron range. About 75% have a Feret’s diameter of <0.1 μm. The largest contribution to the integral particle volume, and thus also to the integral weight, comes from particles with a diameter >1 μm. The size of the particles is probably closely related to their surface charge and residence time. The phase composition varies between drains and condensates. Crystalline phases, such as magnetite, hematite, and lepidocrocite, have been observed in both cases. In condensates, there is a 50/50 relationship by weight between crystalline and amorphous particles, but in drains, crystalline particles are dominant. The reason for this difference is likely a much faster phase transformation at the higher temperatures in preheater drains. A high abundance of magnetite has been found in low-temperature areas such as the condenser. This shows that the Schikorr reaction, favored by high temperature, has little importance in the overall magnetite formation in BWRs.