Leachates collected at two (active and closed) municipal solid waste (MSW) landfills were examined for colloids and particles by transmission electron microscopy, energy dispersive spectrometry, selected area electron diffraction and for the chemical compositions of the filtrates after the filtration to 0.1 µm and ultrafiltration to 1 kDa (~ 1 nm). Six groups of colloids/particles in the range 5 nm to 5 µm were determined (in decreasing order of abundance): carbonates, phyllosilicates (clay minerals and micas), quartz, Fe-oxides, organics and others (salts, phosphates). Inorganic colloids/particles in leachates from the active landfill predominantly consist of calcite (CaCO(3)) and minor clay minerals and quartz (SiO(2)). The colloids/particles in the leachates from the closed landfill consist of all the observed groups with dominant phyllosilicates. Whereas calcite, Fe-oxides and phosphates can precipitate directly from the leachates, phyllosilicates and quartz are more probably either derived from the waste or formed by erosion of the geological environment of the landfill. Low amounts of organic colloids/particles were observed, indicating the predominance of organic molecules in the 'truly dissolved' fraction (fulvic compounds). Especially newly formed calcite colloids forming particles of 500 nm and stacking in larger aggregates can bind trace inorganic contaminants (metals/metalloids) and immobilize them in landfill environments.