It has been previously observed that colloidal particles near a planar electrode can be made to move tangentially along the electrode surface in the presence of other particles by applying an ac-electric field directed normal to the electrode surface. At low frequencies were Faradaic reaction occur (< 1000 Hz) Particles have been shown to aggregate in certain electrolytes and separate in others. Previous work has shown that an electrolyte dependent phase angle between the applied electric field and the particle height could account for this electrolyte dependent behavior. In a model, Fagan et. al. showed that when this phase angle is less than 90º, separation will occur, and aggregation will result when the phase angle is greater than 90º. In this contribution, phase angles were measured on single particles in various electrolyte-electrode combinations. The separation distances were measured on particle doublets as a function of time. In all instances, the experimental observations agree with the doublet motions, either aggregating or separating, as predicted by the phase angle measurements. Aggregation and separation experiments in systems of many particles under the influence of low frequency ac-fields are also presented.