Metal oxide and oxyhydroxide nanoparticles are important components of natural aqueous systems and have application in photocatalysis. Uncoated (oxyhydr)oxide nanoparticles can form charge-stabilized colloids in water, but the precise regimes of dispersion and aggregation have been determined for very few nanomaterials. We studied the colloidal behavior of similar to 6 nm nanoparticles of iron oxyhydroxide (FeOOH), a common natural nanoscale colloid, and found that these nanoparticles formed stable suspended clusters under a range of aqueous conditions. Light and X-ray scattering methods show that suspended fractal nanoclusters are formed between pH 5 and 6.6 with well-defined maximum diameters that can be varied from 25 nm to approximately 1000 nm. The nanoclusters retain a very high surface area, and persist in suspension for at least 10 weeks in solution. The process is partially reversible because optically transparent suspensions are regained when nanoparticles that aggregated and settled at pH > 7 are adjusted to pH 4 without stirring. However, completely redispersed nanoparticles are not obtained even after one month. Because nanocluster formation is controlled predominantly by surface charge, we anticipate that many metal oxide and other inorganic nanoparticles will exhibit equivalent cluster-forming behavior. Our results indicate that natural nanoparticles could form stable nanoclusters in groundwater that are likely to be highly mobile, with implications for the long-range transport of surface sorbed contaminants. (c) 2007 Elsevier Inc. All rights reserved.