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Atomic force microscopy (AFM) in tapping mode was used to determine the conformation of humic substances and aquatic colloids from rivers in an urban catchment in the West Midlands, U.K. Humic macromolecules were shown to have a size of about 1–3 nm in agreement with the literature, indicating that the preparation methods and the AFM were both performing satisfactorily. Three types of natural aquatic colloids were observed by AFM. Firstly, a surface coating about 1–5 nm thick, likely composed of organic and oxide material flattened by drying and interaction with the AFM tip. Secondly, small irregular, globular material between 1 and 70 nm in size, again most likely made of oxide and organic material. Lastly, fibrillar material was present which was 1–10 nm in diameter and 10–1000 nm in length. Most likely this material was microbially produced (muco-) polysaccharides. Size distributions of colloids from all samples, regardless of sample site and sample preparation, indicated colloids with a fairly low polydispersity and with particle numbers dominated by material <10 nm. [ABSTRACT FROM AUTHOR]