A repository of 59 organic compounds has been established by the U.S. Environmental Protection Agency (EPA) for use as gas standards in performance audits during field validation of emission concentrations from stationary sources. These compounds are listed in Title III of the 1990 Clean Air Act Amendment. The compounds are prepared in compressed gas cylinders and recyclable aluminum containers. Three concentration ranges were selected: low (20 to 200 ppb), mid-level (0.5 to 5 ppm), and high (5 to 50 ppm). The recyclable aluminum containers were only prepared in the low range, and pressures were generally below 400 psig. The compressed gas cylinders contained pressures up to 2000 psig. In this program to ensure that the concentration of each gas standard had not changed, each standard was analyzed periodically for stability. The gas mixtures were analyzed by the manufacturer before shipment. They were then analyzed upon receipt, and reanalyzed periodically to determine any change in concentration. The stability data obtained to date indicates that many compounds are stable in the compressed gas cylinders; however, some of the compounds in the recyclable containers are not stable.