Liquids or suspensions are dispersed into sprays of small droplets by atomization of two-phase gas-liquid mixtures. Thus narrow droplet diameter distributions and a high interface area density of the liquid phase are generated in order to increase heat and mass transfer. The mean droplet diameter of the spray is time dependent. It also depends on the total pressure upstream from the nozzle, the volumetric flow rates of the liquid and the gas phase, as well as on the flow regime inside the nozzle. The radial and axial profiles of the void fraction inside the nozzle are measured with an electrical measurement technique. In addition, the flow in the nozzle is visualized by a high-speed camera. Three flow regimes are identified. A model is established to predict their flow regime inside the atomizer. It turns out that the flow regime changes by accelerating the flow to critical conditions. The visualized flow fields are compared to calculated ones. A model to predict the breakup regime is established considering the phase distribution and the critical flow conditions at the exit cross section.