The EPA Method TO-15 Supplement provides supplemental guidance for EPA Method TO-15 as listed in the EPA Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air. The use of gas chromatography/mass spectrometer systems to lower the TO-15 performance criterion for method detection limit (MDL) from 0.5-parts-per-billion by volume (ppvb) to 6 parts-per-trillion by volume (pptv) is described for the compound 1,1-dichloroethene (1,1-DCE). The replicate precision (25%) and audit accuracy (30%), stated as additional performance criteria in TO-15, are retained in the TO-15 Supplement. Samples of 1,1-DCE in the range 30pptv to 5 ppbv as well as samples with only humidified zero air were provided as unknowns to four laboratories. Using methods similar or identical to the method described in the TO-15 Supplement, the analytical results met the 10-15 Supplement's performance criteria with one exception.