Abstract: A system is provided for determining the concentration of a condensable vapor in the presence of at least one other condensable vapor, such as hydrogen peroxide and water vapors. The system includes generally a dew point sensor, a device for measuring at least one additional property of at least the other vapor such as relative humidity, and a microprocessor for fitting the dew point and additional property measurements into a modeled construct representative of the relationship between the vapors at a plurality of concentrations for each such vapor and at a plurality of corresponding dew point measurements. When used to determine the concentration of sterilant vapor in a sterilization system, the system also includes a sterilization chamber and an injector for injecting a measured quantity of the multicomponent condensable vapor into the chamber.