A new gas/particle diffusion separation sampler was developed with the purpose of definitely measuring semivolatile organic compounds (SOCs) in atmospheric gases and particles. A laminar flow separator is able to separate a known fraction of the gas phase from the aerosol because gases diffuse orders of magnitude faster than particles. This fraction of the gas phase is collected downstream of the separator, extracted with supercritical fluid, and analyzed by gas chromatography/mass selective detection. The particulate phase is the difference between measured total and gas-phase concentrations. The separator performance agreed well with theory. Ultimately, we plan to use several separators in parallel to collect semivolatile organic compounds to further the understanding of gas/particle partitioning and the fate of semivolatile species in the atmosphere.