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A series of experiments were conducted among the laboratories participating in the Integrated Atmospheric Deposition Network (IADN) monitoring program to evaluate comparability of the reported persistent organic pollutant concentrations. This quality assurance activity is essential because a variety of methods are currently used for sample collection, extraction, and analysis by the IADN laboratories. The experiments included analyses of a common reference standard (CRS), analyses of split samples, and analyses of samples collected with co-located samplers at the Point Petre IADN measurement station. The analytes included polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs), and polychlorinated biphenyls (PCBs). For virtually all compounds, the laboratories produced generally comparable results for the CRS samples, the split samples and the co-location samples, although some differences were observed. Analysis of the methods may pinpoint areas where variations in the methods will result in the differences observed in the reported data. These differences can be due to the field sampling process, the analytical method, field blank values, or a combination of all these factors. This study points out the importance of QA activities at every step of an environmental monitoring process so that areas where improvements may be needed or where inconsistencies may exist can be identified.