Our breath contains an intraoral and an alveolar part, the latter being the last to be exhaled. As such, major differences in composition can be expected, with for alveolar air the inclusion of more metabolic compounds released during the blood-air exchange. This study explored the differences between breath air from the oropharynx and from the lungs in healthy volunteers. Mouth and alveolar air of 40 volunteers was analyzed using gas chromatography-mass spectrometry, together with a sample of room air. A chromatogram of a single person contains on average 145 different compounds. Altogether, 618 compounds were detected. A total of 35 compounds were common to all volunteers. The number of compounds was significantly higher in mouth air than in alveolar air. For 47 compounds, very significant differences in detection/concentration were recorded. Gas chromatography-mass spectrometry offers a detailed overview of the compounds in a person's breath. One should be aware of the distinction between alveolar air and oropharyngeal air.