Standard face masks are widely used for patients who require high oxygen concentrations. A mechanical model was designed to reproduce various respiratory manoeuvres while oxygen was delivered through a standard variable-performance face mask. Over a range of tidal volumes from 300 to 1200 ml and of frequencies from 10 to 30 breaths/min, the received oxygen concentration was lower than the delivered concentration. The disparity between delivered and received oxygen increased with increasing rate and depth of simulated breathing but could be reduced by increasing the flow rate up to 60 l/min. When a turbulent environment was created, the difference between delivered and received oxygen concentrations was exaggerated. Disparities between delivered and received concentrations can be lessened by increasing the flow rate.