A, method of fragmenting nanoparticle agglomerates by impaction from the aerosol phase in a single-stage impactor is described. The degree of fragmentation as a function of impaction velocity is determined by TEM image analysis of impacted agglomerates. Images of unfragmented particles are obtained by diffusional deposition in the same apparatus but operated in a different way. As an illustration, the method was applied to aerosols of silver, nickel and titanium dioxide agglomerates with primary particle diameters of between 3 and 8.3 nm (Ag), 4 mn (Ni) and 95 run (TiO2), respectively, for which we determined the influence of impact energy and primary particle size on fragmentation. For silver, the degree of fragmentation at a given impact energy decreased with primary particle size; at 3 nm hardly any fragmentation was observed. At a given primary particle size, however, the range of energies required for complete fragmentation was found to-be relatively narrow, indicating a fairly uniform bond structure. These trends are reflected by a model that was devised to calculate bond strength distributions between polydisperse primary particles assuming there are only van-der-Waals interactions. Some measurements were also carried out to investigate potential effects of impact energy on primary particle size. (C) 2003 Elsevier Science Ltd. All rights reserved.