High-flow nasal cannula (HFNC) oxygen therapy has recently shown clinical benefits in hypoxemic acute respiratory failure (ARF) patients, while the interest of non-invasive ventilation (NIV) remains debated. The primary endpoint was to compare alveolar recruitment using global end-expiratory electrical lung impedance (EELI) between HFNC and NIV. Secondary endpoints compared regional EELI, lung volumes (global and regional tidal volume variation (TV)), respiratory parameters, hemodynamic tolerance, dyspnea and patient comfort between HFNC and NIV, relative to face mask (FM).A prospective randomised cross-over physiological study was conducted in patients with hypoxemic ARF due to pneumonia. They received alternately HFNC, NIV and FM.Sixteen patients were included. Global EELI was 4083 with NIV and 2921 with HFNC (p=0.4). Compared to FM, NIV and HFNC significantly increased global EELI by 1810.5 (95%CI: (857; 2646)) and 826 (95%CI: (399.5; 2361)) respectively. Global and regional TV increased significantly with NIV compared to HFNC or FM, but not between HFNC and FM. NIV yielded a significantly higher SpO2/ FiO2 ratio compared to HFNC (p=0.03). No significant difference was observed between HFNC, NIV and FM for dyspnea. Patient comfort score with FM was not significantly different than with HFNC (p=0.1) but was lower with NIV (p=0.001).This study suggests a potential benefit of HFNC and NIV on alveolar recruitment in patients with hypoxemic ARF. In contrast with HFNC, NIV increased lung volumes which may contribute to overdistension and its potentially deleterious effect in these patients.