A vaporizer (14) supplies hydrogen peroxide and water vapor to a high level disinfection or sterilization region (20). Mid-infrared light detectors (32) detect light in the mid-infrared range which has traversed a region of the treatment chamber in a first narrow spectrum centered at 8,000 nanometers which is absorbed by the hydrogen peroxide vapor, a second narrow spectrum which is absorbed by the water vapor, and a third spectrum that is absorbed by neither the hydrogen peroxide vapor nor the water vapor. From these measurements, an absorbance or transmittance is determined (40) from which the concentrations of hydrogen peroxide and water vapor are determined (42). Monitored temperature (44) and the determined concentrations are converted into a percent saturation (54). The supply of hydrogen peroxide and water vapor to the chamber is controlled (60) in accordance with the determined percent saturation.