Abstract: This invention relates to improvements in the method of controlling bio-decontamination cycles used for the bio-decontamination of enclosed spaces, such as pharmaceutical clean rooms, isolators and hospital wards. The bio-decontamination cycle comprises a number of phases including at least one gassing phase, during which sterilant vapour is generated and circulated within the enclosed space. The method is characterised by the steps of continuously measuring the modified relative humidity of the air in the enclosed space, the modified relative humidity being the ratio of water and sterilant vapour: capacity of water and sterilant vapour in the air, and using the measured modified relative humidity to control the steps of the process.