Four sulfonamide veterinary drug residues were quantitated by electron-capture detection (ECD) after separation by gas chromatography (GC). The identities of sulfathiazole (ST), sulfamethazine (SM), sulfachloropyridazine (SCP), and sulfadimethoxine (SDM) were confirmed in bovine or swine liver residues by tandem mass spectrometry (MS/MS). Bovine or swine liver tissues were extracted by using either the Tishler or the Manuel-Steller cleanup. The methylated residues containing ST, SM, SCP, and SDM were separated by GC prior to MS/MS daughter ion analysis. Control tissue, control tissue fortified at 0.1 ppm, and incurred tissue residues at approximately 0.1 ppm were analyzed for these 4 sulfonamides. A Finnigan Model TSQ-46 operating in the chemical ionization mode was used to perform the MS/MS daughter ion experiments. The identities of all 4 sulfonamides were confirmed in a single GC/MS/MS analysis.