The fouling of glass surfaces by nanoparticles formed from corroding iron was studied. Simple experiments demonstrated that adherent films of nanoparticles were deposited by corrosion of iron particles on a glass surface and by flowing water past corroding particles and then onto a glass surface. The water collected from this experiment was found to contain about 2 parts per million (ppm) of particles 500 nm in diameter when tested by photon correlation spectroscopy. However, electron micrographs showed that the primary particles in the fouling film were 20 nm in diameter. This discrepancy was explained by a new theory of nucleation of the fouling films. The theory was confirmed by measuring the particle size of ferric hydroxide dispersions as a function of concentration and pH. It was shown that the 20 nm primary nanoparticles nucleated much larger stable aggregates (defined as nucleags), which were sensitive both to pH and to magnetic fields. In particular, as the pH rose above 6, flocculation occurred, and large unstable agglomerates were observed. The conclusion was that three types of particle could exist in the corrosion product of iron and water: nanoparticles, nucleags, and flocs.