The wall -potential (w), the potential at the shear plane of the electric double layer, depends on the properties of the BGE solution such as the valence and type of electrolyte, the pH and the ionic strength. Most of the methods estimate (w) from measurements of the EOF velocity magnitude u(eo), usually spatially averaged over the entire capillary. In these initial studies, evanescent-wave particle velocimetry was used to measure u(eo) in steady EOF for a variety of monovalent aqueous solutions to evaluate the effect of small amounts of divalent cations, as well as the pH and ionic strength of BGE solutions. In brief, the magnitude of the EOF velocity of NaCl-NaOH and borate buffer-NaOH solutions was estimated from the measured velocities of radius = 104 nm fluorescent polystyrene particles in 33 m fused-silica microchannels. The particle -potentials were measured separately using laser-Doppler micro-electrophoresis; (w) was then determined from u(eo). The results suggest that evanescent-wave particle velocimetry can be used to estimate (w) for a variety of BGE solutions, and that it can be used in the future to estimate local wall -potential, and hence spatial variations in zeta(w.)