A new procedure based on scanning ion microprobe mass analysis (SIMS) is developed to characterize populations of silver halide microcrystals present in a photographic emulsion with emphasis on the thickness distribution of the surface layers usually found on these microcrystals in the 5−30 nm range. Using a Cameca IMS-4f instrument one is able to image the lateral and in-depth distributions of the halides in individual microcrystals. A digital image processing system interfaced with the SIMS instrument permits the acquisition of spatially resolved mass-selected data (ion images) for a number of single microcrystals. Small regions (∼300 nm in diameter) within each crystal are selected a posteriori for local area “retro” depth profiling by computer reconstruction. By averaging the gray values measured in suitable parts of the crystal, the thickness of the surface layers can be determined, allowing one to make a three-dimensional reconstruction of the chemical composition of the microcrystals. Using this methodology, the characteristics (thickness and chemical composition) of surface and subsurface layers of different samples can be compared and subclasses in one population can be detected. With the aid of home-developed hardware and software, the image acquisition procedure was automated, making it possible to automatically measure a sufficiently large number of microcrystals. In this way the analysis time was decreased and the statistical relevance of the data improved. Two examples are given which illustrate the power and practical use of the developed methodology.