A new three-electrode Hall-effect geometry has been used to determine an approximate mobility value for electrons photoemitted into the conduction band of thermally grown layers of silicon dioxide. The oxide layers were grown in an air-steam atmosphere at 1100-1150°C and were from 3.5 to 6.6 μ thick. The unusual electrode geometry was used in order to keep the electron drift path in the oxide less than the Schubweg (mean drift length before immobilization by deep trapping). The geometry is not easy to analyze, and only approximate analyses have thus far been obtained. The observed effect is in all cases of the correct polarity for electrons, and to a good approximation varies linearly with magnetic field strength. The values of electron mobility obtained from the measurements range from 6.8 to 62.5 cm2/V-sec with an average value of about 29 cm2/V-sec.