To assess the long-term environmental impact of radioactive contamination of ecosystems, information on source terms including radionuclide speciation, mobility and biological uptake is needed. A major fraction of refractory radionuclides released from nuclear sources such as nuclear weapons tests and reactor accidents is present as radioactive particles, whilst radionuclides are also present as colloids or low molecular mass species in effluents from nuclear installations. Low molecular mass species are more mobile (lower apparent K(d)) and bioavailable (higher apparent BCF) than colloids and particles. Soils and sediments act as sinks for particles and colloids. Due to particle weathering, associated radionuclides can be remobilised over time. Thus, information on particle characteristics such as composition, particle size, crystalline structures and oxidation states influencing weathering rates and subsequent mobilisation is essential. The present article summarises current knowledge on radioactive particles released from different sources, and the relevance of radionuclide speciation for mobility and biological uptake.