Aim: High-flow-oxygen-therapy is provided by various techniques and patient interfaces, resulting in various inspired-fraction of oxygen (FiO2) and airway-pressure levels. However, tracheal measurements have never been performed.Methods: Three oxygen-delivery-devices were evaluated: 1) standard-high-flow-oxygen-facemask with reservoir-bag, 2) Optiflow(TM)-high-flow-nasal-cannulae and 3) Boussignac(TM)-oxygen-therapy-system. Main judgment criteria were airway-pressure and FiO2 measured in the trachea. The three devices were randomly evaluated in cross-over in 10 Intensive-Care-Unit patients using three oxygen flow-rates (15, 30 and 45 L/min) and two airway-tightness conditions (open and closed mouth). Airway-pressures and FiO2 were measured by a tracheal-catheter inserted through the hole of a tracheotomy tube. Comfort was evaluated by self-reporting. Data are presented as median [25-75th].Results: 1) Regarding oxygen-delivery devices, BoussignacTM provided the highest mean tracheal pressure (13.9 [10.4-14.5] cmH20) compared to Optiflow(TM) (2 [1-2.3] cmH2O, P<0.001). BoussignacTM provided both positive inspiratory and expiratory airway-pressures, whereas Optiflow(TM) provided only positive expiratory airway-pressure. Reservoir-bag-facemask provided airway pressure close to zero. For FiO2, highest value was obtained for both Optiflow(TM) and facemask (90%) compared to Boussignac(TM) (80%), P<0.01. 2) Regarding oxygen-flow, airway-pressure and FiO2 systematically increased with oxygen-flow with the three devices except airway-pressure for the facemask. 3) Regarding the open-mouth position, mean airway-pressure decreased with Optiflow(TM) only (2 [1.2-3.3] vs. 0.6 [0.3-1] cmH2O, P<0.001). Opening the mouth had little impact on FiO2. 4) finally, discomfort-intensities were low for both Optiflow(TM) and reservoir-bag-facemask compared to Boussignac(TM), P<0.01.Conclusion: On one hand, Boussignac(TM) is the only device that generates a relevant positive-airway-pressure during both inspiration-and-expiration, independently of mouth-position. Optiflow(TM) provides a low positive-airway-pressure (<4 cmH2O), highly dependent of mouth-closing. The reservoir-bag-facemask provides no positive-airway-pressure. On the other hand, FiO2 are slightly but significantly higher for Optiflow(TM) and reservoir-bag-facemask than for Boussignac(TM). Discomfort was lesser for Optiflow(TM) and reservoir-bag-facemask.