Halothane samples taken from freshly opened bottles and after long term clinical use in a copper kettle vaporizer were subjected to gas chromatography and mass spectrometry analysis. Of the 3 halogenated impurities found, 2 were identified as cis or trans 1,1,1,4, 4,4-hexafluoro-2, 3-dichloro-2-butene and 1,1,1, 4, 4, 4-hexafluoro-2-bromo-2-butene. While the third impurity could not be positively identified because of its very low concentration, mass spectrometer peaks corresponding to CF3, C3, F3, and Cl were obtained. 1,1,1, 4, 4, 4-hexafluoro-2, 3-dichloro-2-butene has been reported having toxic properties. No information is available concerning the toxicity of 1,1,1,4,4,4-hexa-fluoro-2-bromo-2-butene. It was found that there was no significant enrichment of either butene in the halothane samples taken from the copper kettle vaporizers after long-term use.