A detailed study of the effects of injectant physical properties on the break-up and atomization of a transverse liquid jet in a supersonic airstream was conducted. The tests were run at Mach 3 with ambient stagnation temperature and stagnation pressure of 2.4 atm. Viscosity and surface tension of the injectant along with the injector diameter and the ratio of the jet to freestream dynamic pressures were individually varied (µ= 1.0 - 59.8 centipoise, σ = 15, 33.5, 73.0 dyne/cm., d = 0.45, 0.96, 1.5 nm., q̅ = 1.20) and their effect on the structure and the atomization processes of the jet were established. The investigation employed a short exposure (9 x 10⁻⁹ sec.) photographic technique to establish the instantaneous structure of the jet in the crossflow. Relatively long exposure (10⁻³ sec.) photographs were obtained to study the time averaged behavior of the jet in the crossflow. Two multi-exposure photographic techniques were used to study the velocities of the surface waves that lead to jet break-up along the windward edge of the jet. By employing the Diffractively Scattered Light Method, the mean droplet diameter resulting from atomization at various transverse and axial locations in the spray plume was investigated. The important results are: 1) jet penetration in the crossflow initially increases with increasing viscosity and then decreases, 2) jet penetration is essentially independent of surface tension, 3) for the cases of moderate viscosity and surface tension (values approximately those of water) wave growth and cross fracture of the jet column of the jet is the main mechanism of breakup and atomization, 4) for high viscosity (µ > 40 centipose) ligament formation is the principal mechanism of atomization, 5) increasing viscosity reduces wave growth on the jet surface, 6) wave speed initially increases with increasing viscosity then decreases, 7) wave speed and liquid clump velocities increase with decreasing surface tension, 8) liquid clump velocity decreases with increasing viscosity and surface tension, 9) wave propagation speed is independent of q̅, 10) mean droplet diameter as the injector diameter decreases (D₃₂ = 14 at x/d = 207.7, y/d = 12, dⱼ= 0.45 mm.), 11) increasing viscosity increases droplet diameter (D₃₂ = 16 at x/d = 93.2, y/d = 12.4 µ = 1 .0 to D₃₂ = 21 at x/d = 93.2, y/d = 10.4, µ = 10.0), 12) decreasing surface tension decreases the droplet diameter (D₃₂ = 14, σ = 73.0 dyne/cm., D₃₂ = 5, σ = 15 dyne/cm.). / Ph. D.