Breath hydrogen sulfide (H2S) and methyl-mercaptan (CH3SH) concentrations are used as quantitative indicators of halitosis. However, measurements of these gases in duplicate oral samplings often show poor reproducibility. To determine if this poor reproducibility is an artifact of the collection/analytical procedure or a true biological phenomenon, we used a standardized technique to collect from 20 to 30 oral gas samples at two-minute intervals from 11 healthy subjects. The samples were analyzed for sulfur gases and CO2. Sizable variations in H2S and CH3SH concentrations were not associated with alterations in CO2, indicating that the variations did not reflect variable contamination with atmospheric or pulmonary gas. In addition, fluctuations in H2S and CH3SH were not identical and often were not random. We conclude that minute-to-minute variability in oral sulfur gas concentrations is a true biological phenomenon. This fluctuation complicates experimental studies designed to show that interventions alter halitosis.