The operating principles and performance of a new type of spray nozzle are presented This nozzle, termed a ligament-controlled effervescent atomizer, was developed to allow consumer product manufacturers to replace volatile organic compound (VOC) solvents with water and hydrocarbon (HC) propellants with air, while meeting the following restrictions: that the spray mean drop size remain below 70 mu m, that the atomizing air consumption be less than 0.009, and stat atomizer performance be uncompromised by the increase in surface tension or by changes in viscosity. The current atomizer differs from previous effervescent designs through inclusion of a porous disk located immediately upstream of the nozzle exit orifice. The purpose of this disk is to control the diameter of ligaments formed at the injector exit plane. Atomizer performance is reported in terms of the spray Sauter mean diameter, with drop size data analysed using a model developed from first principles. The model describes the spray formation process as the breakup of individual cylindrical ligaments subject to a gas stream. Ligament breakup length is obtained using the expression of Sterling and Sleicher [1]. Ligament diameter is estimated frost manufacturer-supplied pore size data for the porous disk. The model correctly predicts the experimentally observed relationship between Sauter mean diameter and air-to-liquid ratio by mass, liquid surface tension, and liquid viscosity.