There is disclosed a method and system for determining the partial pressure of at least one gas in a mixture of gasses contained in a pressure vessel, in particular a pressure vessel in the form of a life support pressure chamber/decompression chamber, or a diving gas storage cylinder. The method comprises the steps of: coupling a gas analysis sensor (14) to a pressure vessel (10)； directing a portion of the mixture of gasses in the pressure vessel to the sensor for analysis； reducing the pressure of the portion of the mixture which is to be analysed by the sensor to a level which is below the pressure in the vessel but above local atmospheric pressure； operating the sensor to measure the partial pressure of the at least one gas at the reduced pressure level； and using the partial pressure of the at least one gas, measured at the reduced pressure level, to determine the actual partial pressure of said gas in the mixture contained in the vessel.
