Knowledge of particle size distributions (PSDs) in seawater is important for understanding several facets of marine science, such as the behavior of light scattering in seawater, phytoplankton dynamics, and biogeochemical cycling. Here, a method has been developed to quantify the size distribution of particle suspensions and characterize their chemical composition utilizing a scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS) and applying image analysis techniques, including automatic thresholding. The method was validated by verifying the PSD and chemical composition of the Arizona Test Dust (ATD), which has a well-documented size distribution and chemical composition. Size distributions of ATD particles containing specific elements important in the marine environment, such as silicon, iron, calcium, aluminum, and potassium, were quantified. PSDs determined with the technique in field samples from coastal Long Island Sound and the remote South Pacific were compared with other sizing methods, including electroresistivity and laser diffractometry. Most accurate results for PSD determinations occurred when the particle mass loading on the filter was between 0.04 and 0.1 mg cm 22. With this in mind, immediate feedback in the field can be provided to prepare appropriate filtration sample volumes due to a linear relationship between the beam attenuation coefficient at 650 nm (c 650) and the total suspended matter (TSM). Overall, the method presents two defining advantages in 1) minimizing user bias, because the majority of the analysis is automated, and 2) providing an elemental distribution in the context of a particle size distribution.