The US Environmental Protection Agency (EPA) published a National Ambient Air Quality Standard (NAAQS) and the accompanying Federal Reference Method (FRM) for PM10 in 1987. The EPA revised the particle standards and FRM in 1997 to include PM2.5. In 2005, EPA proposed revisions to this NAAQS to include PM10–2.5 but only finalized revisions with a PM2.5 FRM and the development of a national monitoring network in 2006. Presently, no EPA designated reference or equivalent method sampler has the ability to directly measure the mass concentrations of PM10, PM10–2.5, and PM2.5 simultaneously. An additive approach has been used for samplers like the dichotomous monitors to calculate PM10 mass concentrations from independent measures of PM10–2.5 and PM2.5 (i.e. PM10 =PM10–2.5 +PM2.5). A differential approach has been used to calculate PM10–2.5 from identical collocated PM10 and PM2.5 samplers (i.e. PM10–2.5 =PM10–PM2.5). Since these two approaches have been used widely for PM measurements, it is informative to evaluate their precision and comparability. EPA performed collocated tests of five different particle samplers in the Research Triangle Park area of North Carolina to evaluate the comparability and to characterize the additive and differential approaches used to determine particle mass concentrations. The intra-sampler precision of MiniVol, Omni, and dichotomous samplers was less than 8.4%. The precision of PM10 measurements using the additive approach with dichotomous samplers was less than 3.5%. The poorest precision of the various PM10–2.5 differential approaches was less than 15.1%. No zero or negative PM10–2.5 concentrations were calculated using the differential approach. A coefficient of determination of 0.81 or higher was obtained for all paired comparison of PM10–2.5. The reported test results show that concentrations calculated from both the additive and differential approaches generally agree among the portable samplers, the more established dichotomous sampler and the Federal Reference Methods