A method has been devised for the determination of potentials of areas on the interior surface of the glass envelopes of vacuum tubes. Measurements by this method may be made on almost any type of finished tubes. Results obtained have been checked by a second method. Determinations of luminescent–screen potential as a function of second–anode voltage have been made for kinescope tubes under different conditions as to type of screen material, manner of bombarding the screen, beam–current density and previous electron bombardment. It was found as a general feature that as the second anode is made increasingly positive, the screen follows and remains a few volts negative with respect to the second anode until it starts to approach an upper limit of potential with respect to the cathode. When this occurs, the screen rapidly becomes more negative with respect to the second anode. Calcium–tungstate screens and clean Nonex glass surfaces have been found to reach upper limits of potentials between 2500 and 5000 volts, while willemite screens have been carried to potentials as high as 20,000 volts. The upper limit of potential for a particular type of screen varies from tube to tube. Values as different as 4000 and 20,000 volts have been obtained for willemite. In general, the upper limit of potential is found to be lowered as an effect of increased current density in the bombarding beam. Aging of a screen has been found to lower permanently its upper limit of potential. Accumulation of negative charges on the glass base of a screen and on depressed surfaces in the screen layer appears to be important in determining the upper limit of screen potential.