A trailer-deployed automated gas chromatograph--mass spectrometer (autoGC-MS) system capable of making continuous hourly measurements was used to determine volatile organic compounds (VOCs) in ambient air at New Hendersonville, Tennessee, and Research Triangle Park, North Carolina, in 1995. The system configuration, including the autoGC-MS, trailer and transfer line, siting, and sampling plan and schedule, is described. The autoGC-MS system employs a pair of matched sorbent traps to allow simultaneous sampling and desorption. Desorption is followed by Stirling engine cryofocusing and subsequent GC separation and mass spectral identification and quantification. Quality control measurements described include evaluating precision and accuracy of replicate analyses of independently supplied audit and round-robin canisters and determining the completeness of the data sets taken in Tennessee. Data quality objectives for precision (¿10%) and accuracy (¿20%) of 10- to 20-ppbv audit canisters and a completeness of >75% data capture were met. Quality assurance measures used in reviewing the data set include retention time stability, calibration checks, frequency distribution checks, and checks of the mass spectra. Special procedures and tests were used to minimize sorbent trap artifacts, to verify the quality of a standard prepared in our laboratory, and to prove the integrity of the insulated, heated transfer line. A rigorous determination of total system blank concentration levels using humidified scientific air spiked with ozone allowed estimation of method detection limits, ranging from 0.01 to 1.0 ppb C, for most of the 100 target compounds, which were a composite list of the target compounds for the Photochemical Assessment Monitoring Station network, those for Environmental Protection Agency method TO-14, and selected oxygenated VOCs. ¿ 1998 American Geophysical Union