The internal structure of materials prepared by aggregation of oppositely charged polystyrene spheres (electrostatic heteroaggregation) is investigated by static light scattering, optical microscopy, and Brownian dynamics simulation Light scattering indicates ultralow mass fractal dimensions, as low as 12 Such low fractal dimensions, approaching the theoretical limit of a linear object, imply a chaining mechanism Optical micrographs reveal linear chains with the particle charge alternating down the chains. Brownian dynamics simulation gives additional support for a chaining mechanism For the polystyrene system (120-nm primary particle diameters), the fractal dimension is found to increase from 1.2 to 17 as the background electrolyte is increased. In terms of electrostatic screening, the results match those reported recently for larger polystyrene spheres. The low fractal dimensions appear to represent a crossover from linear chains to a structure of diffusion-limited aggregates, however, experiments under density-neutral conditions imply that sedimentation plays an important role in the formation of ultralow fractal dimensions The practical implication is that microcomposites with a locally uniform distribution of starting materials and almost any degree of branching can be prepared from oppositely charged particles. (C) 2003 Elsevier Science (USA). All rights reserved.