question: Purpose: In the context of the COVID-19 pandemic, mandatory ventilation with noninvasive ventilation (NIV) devices is a valid option when intensive care/anesthesia ventilators are unavailable. The fraction of delivered oxygen (FDO 2 ) by NIV devices in intubated patients is unknown. Method: We simulated intubated patients with normal and sick lungs. NIV was used in pressure control mode with protective lung ventilatory settings. O 2 flow was added into the NIV circuit in incremental steps of 1 L/min (from 1 to 15 L). The FDO 2 in breathing gases was measured by a paramagnetic O 2 sensor placed behind the endotracheal tube. Three NIV circuit options were analyzed: 1) leak at HME filter close to the patient, 2) anesthesia Bain circuit with leak distal to the patient, and 3) leak throughout a non-rebreathing valve near the patient. Results: : FDO 2 increased proportional to the supplemental O 2 flow in all NIV options and in both kinds of patients. The range of FDO 2 came from 0.25 to 0.98 in both, healthy and sick lungs. At 5 L/min, FDO 2 was 0.53±0.04 and 0.47±0.02 in option 1 and 0.53±0.04 and 0.47±0.02 in option 2 for healthy and sick lungs, respectively. In option 3, 5 L/min of O 2 reached 0.84±0.08 in healthy and 0.74±0.09 in sick lungs. Conclusions: : In all setups, FDO 2 was proportional to the administered O 2 flow and it covered the range of FDO 2 values commonly observed in ventilated patients.