The present invention provides a method and apparatus for sterilizing articles using an ozone-containing gas, where condensation of water from the sterilization atmosphere during the sterilization process is substantially prevented. The inventive sterilization method includes providing a sterilization chamber and placing an article into the sterilization chamber. The sterilization chamber is sealed prior to equalizing the temperature of the article and the atmosphere in the sterilization chamber. A vacuum is applied to achieve a preselected vacuum pressure in the sterilization chamber. Once the vacuum pressure is set, water vapour is supplied to the sterilization chamber. Ozone-containing gas is then supplied to the sterilization chamber and the sterilization chamber remains sealed for a preselected treatment period, where the sterilization chamber remains sealed throughout the whole process. Finally, vacuum in the sterilization chamber is released.