Dynamic light scattering was used to study the Brownian translational diffusion and rate of Brownian aggregation of Laponite (RD) clay particles at low (millimolar) electrolyte concentrations. Laponite is a manufactured clay consisting of monodisperse disk-shaped particles with a 30-nm diameter and a 1-nm thickness. The stability ratio, defined as the ratio of the coagulation rate for Brownian spheres with no particle interactions to the observed coagulation rate, was quite large O (10(5)), suggesting that there was a large potential energy barrier to Brownian aggregation. The apparent potential energy barrier for face-edge aggregation was rationalized on the basis of a calculation of the electrostatic interactions between two disks with negative face charges and positive rim charges. The aggregation rate increased with increasing electrolyte concentration owing to the screening of the electrostatic repulsion associated with the net charge on the particle. The rate decreased with increasing pH because of the decreasing positive charge on the rim. The translational diffusivity of the individual particles before the onset of aggregation exhibited a strong dependence on the electrolyte concentration and was as much as 50% smaller than the diffusivity for an uncharged disk. This effect is attributed to the added drag resulting from the electroviscous effects in the deformed double layer. The electroviscous effect on the diffusion of the disk-like particles is much stronger than that on rods and spheres.