The accuracy and precision of U.S. Environmental Protection Agency Reference Method 23 was evaluated at a trichloroethylene degreasing facility and an ethylene dichloride plant. The method consists of a procedure for obtaining an integrated sample followed by gas chromatographic analysis. This study identified a number of conditions such as sunlight and sample volume that may affect the performance of the Method. A paired sampling technique was employed to obtain identical samples, from which the precision and variable interactions could be determined. Certain results fell outside the range of acceptable precision, especially when mylar bags were employed. Modifications to the method are suggested that specify that only Tedlar bags be used for sampling, that incorporate an on-site technique for leak detection, and that discontinues the procedure for moisture removal. With these modifications, Reference Method 23 was fully validated as acceptable for field use.