of a volume of 125 litres 4 cubic feet at this stage the sterilization chamber is still under the effect of a partial vacuum of about 670 mbar in an optional second step the pressure level is raised to about 900 mbar using oxygen as a filing gas this pressure level is maintained for about 20 mm after the sterilization period the vacuum is reapplied preferably at a pressure of about 1 0 mbar again once the vacuum reaches 0 1 mbar the humidification phase is recommenced followed by the renewed injection of an oxygen ozone sterilization gas mixture followed by the sterilization period the cycle of applying a vacuum of about 1 0 mbar injecting sterilization gas humidifying and sterilizing can be repeated and the number of repeat sterilization cycles mini cycles selected to achieve complete sterilization of the instruments preferably between any two successive sterilization cycles a re conditioning step is effected as described above to remove any condensed water from the sterilization chamber the number of repeat cycles used in an experimental set up of a method in accordance with the invention including a 125 litres 4 cubic foot chamber was 2 repeat cycles this set up conformed to the security assurance level standards of the fda sal 10 6 to remove all remaining ozone and humidity in the sterilization chamber 10 after complete sterilization after all successive sterilization cycles a ventilation phase is engaged the.