Acid rain occurs when sulfur dioxide and nitrogen oxide emissions are transformed in the atmosphere and return to earth in rain, fog or snow. Most of the emissions are from the burning of fossil fuels by electric utilities. The electric utilities must reduce their emissions to comply with the new Clean Air Act Amendments of 1990. The utilities continuously monitor their emissions by using Continuous Emissions Monitors (CEMs). Compressed gas standards are used for calibration and audits of CEMs. The specialty gas manufacturers of these compressed gas standards must prepare the standards according to an EPA traceability protocol that was originally published in 1978. An accuracy assessment study of specialty gas manufacturers' protocol gases containing SO2 and NO was conducted in 1991. These same cylinders have been reanalyzed to evaluate the stability of the protocol gases. The paper presents the results of the stability study of compressed gas cylinders over a two-year period. The protocol gases were found to be stable within + or - 2% over that period.