The performance of an air-assist pressure-swirl atomizer and, more important, its limitations are described. The design of the present atomizer is based on a pressure-swirl nozzle, but differs from conventional single-phase pressure-swirl designs in that the liquidfiltn in the exit orifice is stabilized by axially injecting air through the upstream plane of the swirl chamber: The present study includes drop size data obtained using a Malvern 2600 HSD particle size analyzer: Atomizer performance was assessed using three different nozzle configurations and four different liquids. The drop size data indicate that an increase in liquid supply pressure, liquid mass flow, rate, or aatomizing air-to-liquid ratio by mass (ALR) leads to a decrease in Sauter mean diameter (SMD). It also shows that spray quality (i.e., mean drop size) is independent of swirl chamber geometry at constant liquid supply pressure and ALR for low viscosity liquids, Atomizer exit orifice diameter has little effect on SMD when operating at constant liquid supply pressure far these same low-viscosity liquids. However, the effects of liquid mass flow rate and exit orifice diameter are coupled with an increase in exit orifice diameter, lending to mt increase ill SMD when liquid mass flow rate is constant The influence of both swirl chamber and exit orifice diameter is enhanced when liquid viscosity climbs to 0.010 kg/m-s. In these cases, SMD increases,vith a decrease in swirl chamber diameter regardless of whether liquid mass flow rate or supply pressure is kept constant. SMD continues to decrease with a decrease in exit orifice diameter for both the constant liquid mass flow rate and constant liquid supply pressure cases. Finally, the data indicate that mean drop size increases with an increase in either liquid viscosity or surface tension. A first principles model was developed to explain the observed SMD scaling with operating conditions, nozzle configuration, and liquid physical properties. It combines the classical inviscid analysis for the flow inside a pressure-swirl atomizer with a correlation for the air-to-liquid velocity slip ratio, a geometric model for ligament formation, and a linear fluid mechanical instability analysis to describe ligament breakup. Model predictions reflect the observed SMD scaling with variations in liquid supply pressure, liquid mass flow rate, ALR, swirl chamber diameter, exit orifice chamber diameter, and surface tension. Accuracy is best for intermediate and high liquid delivery rates, Viscosity scaling is not captured accurately, That lack is ascribed to the inviscid internal flow model employed here, which cannot account for rite increase in liquid film thickness with an increase in liquid viscosity.