A method for determining and sterilizing a load in a sterilization chamber. A small amount of sterilant is introduced into the sterilization chamber and the concentration of sterilant is measured. The load of equipment to be sterilized is determined from the concentration of sterilant, and more sterilant is added, if necessary, based on the load. The process is repeated, if necessary, until the load is sterilized. The sterilant is preferably hydrogen peroxide, and the concentration of hydrogen peroxide is preferably determined by a spectrophotometric method in the infrared or ultraviolet regions. By monitoring the sterilant concentration and adding more as needed, the equipment in the chamber can be sterilized efficiently without exposing it to high concentrations of sterilant which could damage the equipment or leave too much residual on the equipment.