A new technique for measuring relative changes in thin film thickness is utilized as a means of monitoring sputtering yields at low energies. The apparatus consists of a Hitachi HU‐10 electron microscope with an ion source attached to it which allows exposure of the specimen to ion bombardment before and after monitoring transmission current of electrons through it. Operating the microscope in the selected area diffraction mode, the electron beam current is monitored by a Faraday cup placed in the microscope at the position of the final diffraction pattern image. Relative changes in thickness as low as 4 Å for 100‐Å thick films are measured. A scanning technique avoids contamination buildup during the electron transmission measurement. The specimens monitored have shown a high degree of uniformity with variations in sample thickness less than the estimated error of the measurement. The sensitivity of this method is limited only by available ion current densities rather than total current to the specimen. In principle, mass losses of 2×10−14g are detectible by the electron beam probe of 2‐&mgr; diameter. The apparatus has been applied to measuring sputtering yields of polycrystalline silver thin films as a function of energy at normal incidence utilizing argon, krypton, and neon as the bombarding ions. The results in an energy range from 30 to 200 ev agree qualitatively with those of Koedam, particularly with regard to the mass variation. The sputtering yields for Kr+ions on Ag is intermediate to that of Ar+and Ne+. At 30 evS≃2×10−4for Kr+on Ag.