This application note will benefit chemical and material manufacturers concerned about micron and submicron par ticle contamination. Outlier s are a common problem in many industrial applications. Outliers can be defined as particles that are significantly larger or smaller than the main par ticle size distribution, but which occur in a m uch lower frequency than the main par ticle size. As a simple e xample, consider a hypothetical sample that contains 1,000,000 particles with a diameter of exactl y 10 microns and onl y one par ticle with a diameter of 40 microns mixed together. In this sample, the 40-micron particle would be considered an outlier to the main distr ibution. Detection of outliers presents a special problem in par ticle characterization, since most instr uments are designed to repor t on the average size of a population, and often miss outliers with lower frequency of occurrence. For particle characterization by light scatter ing, the data can also be highl y skewed due to a few large outliers. Larger outliers may dominate the signal because light scatter ing scales with (radius of particle) to the sixth power. This application note demonstr ates the ability of the Coulter Pr inciple to detect outlier s in suspensions of highly concentrated small particles.