Discharge coefficients for plain-orifice effervescent atomizers are measured over wide ranges of air/liquid ratio, ambient air pressure, air injection pressure, and liquid properties. Three different discharge orifice diameters of 1.2, 1.6, and 2.0 mm are employed in conjunction with a constant mixing chamber diameter of 6.4 mm. The results obtained show that discharge coefficients are governed primarily by air/liquid mass ratio and air/liquid density ratio, with liquid properties playing only a minor role. It is also found that continuous increase in air/liquid mass ratio leads eventually to flow instabilities and a pulsating spray. The range of air/liquid ratios over which the spray remains stable can be extended by increases in flow velocity and/or air/liquid density ratio. An equation for estimating the maximum air/liquid mass ratio at which stable atomizer operation can be sustained is derived.