Abstract: The present invention relates to an arrangement for the analysis of respiratory gases to and from a patient, which is connected to a respirator. The arrangement comprises a holder unit (6) for a removably fitted water trap (4) that is adapted to receive said respiratory gases and the arrangement has a connection that is adapted to lead liquid-free gas from the water trap (4) to an analysing instrument (8). The holder unit is provided with an oxygen gas measuring unit (14) for measuring oxygen gas in the liquid-free gas which is removably attached to said holder unit (6). The present invention also relates to the above mentioned oxygen gas measuring unit (14).