To attempt a better understanding of the mechanism of ion detection with ion‐sensitive plates, a study was made of sensitivity as a function of ion energy, and of blackening as a function of exposure. Various commercially available and experimental plates used in mass spectrography were investigated. A dc beam of Cs+ions was used to expose evenly areas 0.1 by 0.25 cm on the plates, which were placed between the ion beam monitor collector and the magnetic sector entrance boundary of an AEI type MS7 double‐focusing mass spectrograph. Thus, after leaving the ion source, the ions traversed only the electrostatic sector portion of the instrument. For the sensitivity study, exposures were made with ion energies ranging from 0.1 to 20 keV on Ilford Q2, Kodak SWR, Kodak‐Pathe´ SC5, and RCA thin‐film plates made by the direct evaporation of AgBr onto glass (described in a companion paper by Honig, Woolston, and Kramer). In addition, blackening as a function of exposure was determined at various energies for each ion‐sensitive material. The relationship observed between sensitivity and ion energy, when plotted in double‐logarithmic fashion, is not a simple linear function, as has been stated by previous investigators, but exhibits a complex behavior.