Aim: In this study, the aim is to determine the effects of BIPAP on alveolar gas exchange and haemodynamics over BIPAP + PSV mode in acute respiratory failure. Material and method: After sedation, 50 patients with acute respiratory failure were ventilated. BIPAP was preferred for first six hours and 10 cmH2O PSV was added on BIPAP for the second six hours. Mean arterial pressure (MAP), heart rate (HR), inspiratuar pressure (Pinsp), peak pressure (Ppeak), plato pressure (Pplato), mean pressure (Pmean), minimum pressure (Pmin),intrinsic PEEP (PEEPi,) total minute volume (MVtop), spontaneous minute volume (MVspo) and ratios, total frequencies (ftop), spontaneous frequency (fspo) and ventilatuar frequency (fmek), inspirium ve expirium tidal volumes (VTinsp, VTeksp), resistans, complians, arteriel blood gases values, FiO2 values and Horovitz ‹ndex (PaO2/FiO2) were recorded at, T1: initiation of ventilation, T2: end of the BIPAP ventilation, T3: end of BIPAP + PSV ventilation. Results: There was no statistically difference in haemodynamic data, Ppeak, MVtop, VTinsp, VTeksp, airway resistance, compliance, PaCO2, SaO2 ve PaO2 values. The increase in spontaneous respiratory rate and decrease in Pinsp with BIPAP + PSV mode was statistically significant (p<0.05). Arterial blood gas analysis with BIPAP + PSV ventilation, FiO2 levels were reduced while Horovitz index values were increased (p<0.05). Conclusion: BIPAP and adding 10 cmH2O PSV provide positive effects on oxygenation. We think that the effects of this kind of hybrid ventilation should investigate at homogenous subgroups and long term ventilation.