In December 1997, Emergencies Science Division (ESD) was contracted by Natural Resources Canada (NRCAN) to perform on-site analyses in support of a transformer-rebuilding project at Sault Ste-Marie, Ont. Using a gas chromatograph with electron capture detector (GC/ECD) mounted in a mobile laboratory, PCB analyses were conducted on the original transformer oil, surface wipes, Varsol rinsing of the transformer tank interior and cooling fins. To assess the efficiency and validity of the decontamination process, PCB contamination was monitored closely on the rinse solvent. Surface wipe samples after wash down showed surface concentration of several hundred mug Aroclor 1254/100 cm(2), well below the acceptable limit of 8000 mug/100 cm(2). Because of the relatively large percentage of the internal surface area, the fin banks had to be rinsed exhaustively to meet the decontamination criteria. Final rinses of each of the seven fin banks of transformer I still showed presence of PCB, ranging from 80 to 590 ppm (mug/ml) with a mean value of 280 ppm. Upon completion of rebuilding, analysis of the R-Temp retro fill fluid showed 5 ppm at the initial power up, increasing slightly to 16 ppm after 1 year of operation, which was far below the regulatory limit 50 ppm The second transformer, by comparison, had a lower mean concentration of 54 ppm in the final fin rinse during decontamination. However, the backfill R-Temp showed an initial concentration of 38 ppm and remained essentially unchanged at 32 ppm after approximately 10 months of operation. Extensive comparison of GC and the quick test Clor-N-Oil kit were also carried out and showed generally good agreement. The use of an on-site GC was crucial in providing rapid and accurate analysis on-sire, thus, enabling quick modifications to the decontamination strategies in order to meet the target PCB level. For projects of this nature, a GC/ECD was far superior to quick test kits by providing the selectivity and sensitivity for the diverse nature of the sample media.