The electrophoretic mobility of quartz in 0.01 mol dm(-3) NaCl and NaNO3 over the pH range 2-8 was studied using five different commercial instruments. The mobilities over the pH range 4-8 were relatively consistent, but the mobilities over the pH range 2-4 and the position of the isoelectric point IEP varied from one instrument to another. This result suggests that the discrepancies in the apparent IEP of quartz (and other silicas) reported in the literature are due to the instrument artifacts.