High velocity, gas-assisted liquid drop atomization processes were investigated under well-controlled experimental conditions at elevated gas densities and room;temperature. A monodisperse stream of drops which are generated by a vibrating-orifice drop generator was injected into a transverse high velocity gas stream. The gas density and air jet velocity were adjusted independently to keep the Weber numbers constant. The Weber numbers studied were 72, 148, 270, and 532. The range of experimental conditions studied included the three drop breakup regimes previously referred to as bag, stretching/ thinning and catastrophic breakup regimes. High-magnification photography was taken to study the microscopic breakup mechanisms in high velocity gas flow fields. When the Weber number is held constant at different gas densities and jet velocities, the results show that the microscopic breakup process is similar, even at high gas densities. At low Weber numbers, the photographs confirmed the existence of the bag breakup regime. The stretching/thinning breakup regime was observed for Weber numbers between 150 and 270, At Weber number = 532, the breakup in the catastrophic breakup regime occurred. (C) 2000 Elsevier Science Ltd. All rights reserved.