Thiols are generally amenable to gas chromatographic analysis, When thiophenol is analyzed by gas chromatography-mass spectrometry, however, its mass spectrum also shows the molecular ion for dipheny 1 disulfide. This paper reports a systematic investigation establishing that the oxidation of thiophenol to dipheny1 disulfide takes place in the GC/MS interface and is affected by heat. Similar phenomena are also reported forp-chlorothiophenol, cyclohexyl mercaptan, andn-hexyl mercaptan.