Abstract Measurements of the electrophoretic mobility (uE) of particles of colloidal -alumina were made as a function of pH, electrolyte concentration and electrolyte type (NaCl, NaNO3 and KCl) using two similar instrumental techniques. Significant differences (50% or less) in the values of uE of particles in NaCl were obtained from the two instruments; however, the isoelectric points (IEPs) (the pH at which uE=0), estimated from the two sets of measurements, occurred at 7.50.3 and 7.80.05 and were not significantly different. The latter estimate corresponds with those for particles in KCl and NaNO3 of 8.050.11 and 7.950.18, respectively, made using the same instrument and indicate that the IEP was a weak function of electrolyte type. When cations acted as counterions (pHIEP), the absolute magnitudes and the ranges of uE with electrolyte concentration were found to be significantly less than when anions acted as counterions (IEPpH). Estimates of the zeta potential (), made using various procedures, showed variations of up to 25% at low ratios of electrical-double-layer thickness (1) to particle radius (a) (a10) and were of a similar scale to differences in uE, but no significant variations (95% confidence) in were obtained at high values (a200).