The separation characteristics of the PM2.5 aerosol size selectors used in speciation samplers developed for the U.S. EPA National PM2.5 Chemical Speciation Trends Network were evaluated under clean conditions. Measurement of particle penetration versus aerodynamic diameter was conducted using an APS 3320 in conjunction with a polydisperse test dust. The resulting penetration curves were integrated with assumed ambient particle size distributions (40 CFR Part 53, Subpart F) to obtain an estimate of measured mass concentration and to predict bias relative to the PM2.5 reference separator. The cutpoint of two sharp cut cyclones, from the family of cyclones developed by Kenny and Gussman (1997), compares favorably with the WINS, although possessing a slight tail that extends into the coarse particle mode. A second cyclone used by the Andersen Corp., AN 3.68, demonstrated the sharpest cut characteristics of the devices tested; however, it possesses a