Investigations have been carried out on the distribution of AgBr-grains in unexposed photographic plates (type Q2, Ilford Ltd.) used for ion detection in mass-spectrometers. Electron micrographs show that a large fraction of the grains protrude about halfway out of the gelatine matrix and that these grains are covered by a very thin gelatine layer on the side facing the ions. The mean projection area of the unexposed grains, the percentage of the plate area covered with unexposed grains, and the emulsion thickness have been determined. The shape of unexposed as well as exposed and developed grains is discussed. A model of the distribution of the AgBr-grains in the emulsion is presented.