The present study investigates the influence of system pressure, gas velocity, and annular gas gap width on the resulting droplet size. Three external-mixing twin-fluid atomizers are operated at a constant liquid mass flow. The nozzle geometry is kept similar, except that the annular gas gap width is changed. At every system pressure level (1 – 21 bar), three different gas velocities were investigated by changing the gas mass flow. High-speed camera images are used for observation of primary breakup and discussed with regard to local measurements of droplet size performed by a phase Doppler anemometer. The gas momentum flux as well as the gas momentum flow were applied to describe the atomization process under varying operating conditions. Finally, an empirical model is derived, enabling the system pressure scaling of external-mixing twin-fluid atomizers for the range of gas momentum flow under investigation.