Abstract: The invention includes a system and a method for maintaining a selected concentration of a sterilant vapor during vapor phase sterilization. The system includes a sterilization chamber with a source of multicomponent vapor containing a sterilant vapor, an electromagnetic radiation source capable of generating radiation at a plurality of wavelengths, a radiation detector for detecting and quantitating the electromagnetic radiation absorbed at the wavelengths, the detector generating absorbance signals proportional to the concentration of the sterilant vapor in the multicomponent vapor, a microprocessor programmed to compare the absorbance signals with reference absorbance signals, to calculate the sterilant vapor concentration therefrom and to generate an output signal, and a controller to receive the output signal and to control addition of the sterilant vapor to the sterilization chamber to maintain a selected concentration of the sterilant vapor in the sterilization chamber during the sterilization process.