There is described a method of use of a mass exchanger. In the method the mass exchanger comprises: a first channel for accommodating flow of a liquid to be treated； and a second channel for accommodating flow of a treatment agent, the first and second channels have a permeable membrane provided between them, so as to allow transfer of selected species between the first channel and the second channel. The steps of the mass transfer method comprise passing the liquid to be treated along the first channel and introducing a mixture of liquid and gas into the second channel to provide a two-phase treatment agent. It is desirable to provide a means of adjusting the concentration of gas species in a liquid such as blood, while simultaneously controlling the temperature of the liquid and optionally adjusting the concentration of ionic and/or dissolved species in that liquid. By this method and mass exchanger providing a two-phase treatment agent, it is possible to simultaneously deliver gaseous species (e.g. oxygen) into the treated liquid, while making use of the high heat capacity of the liquid phase of the treatment agent to transfer significant heat into or from the treated liquid.
