A method for the determination of the carbon dioxide content of an exhaled gas stream is provided, the method comprising measuring the water vapour content of the exhaled gas stream,- and determining the concentration of carbon dioxide in the exhaled gas stream from the measured water vapour content. A particular method comprises causing the gas stream to impinge on a sensing element comprising a working electrode and a counter electrode; applying an electric potential across the working electrode and counter electrode; measuring the current flowing between the working electrode and counter electrode as a result of the applied potential; and determining from the measured current flow an indication of the concentration of the water vapour in the gas stream. A sensor is also described.