A method of sterilisation, decontamination and/or sanitation of an enclosed environment, the method comprising the steps of measuring the temperature of an enclosed environment to be treated; calculating the relative humidity required to provide a desired partial pressure at the measured temperature, the actual desired partial pressure required being based on a predetermined optimum partial pressure at a base temperature corresponding to an optimum operating temperature for that environment; introducing a predetermined quantity of water to the environment to provide the calculated relative humidity for the measured temperature; discharging ozone into the humidified environment; maintaining the ozone level at a concentration that will achieve the required degree of decontamination, sterilisation and/or sanitation of the humid environment; and reducing the ozone level to an acceptable safe exposure level.