The objective of this study is to characterize the operation of an air-assisted fuel injector. This characterization involves four sets of tests: fuel and air flow calibration; instantaneous measurements of fuel and air solenoid signals, internal pressure in the injector, and poppet lift; photographs of the spray; and droplet sizing. The injector poppet was designed to form a spray of 80° included angle. Nitrogen, instead of air, was used to assist the injection of unleaded gasoline into steady, compressed nitrogen at room temperature. The following conditions were used: nominal fuel flow rates of 10, 20, and 30 mm3/injection; spray chamber pressures of 0.1, 0.169, and 0.445 MPa; and nominal injections per minute (IPM) of 1600 and 3000. Results showed a linear increase in total fuel mass supplied to the injector as fuel solenoid pulse width was increased, except at the highest IPM and chamber pressure when the total fuel mass tended to level off. The mass of fuel injected showed a linear increase with fuel solenoid pulse width while operating at 1600 IPM, but it tended to level off at 3000 IPM. Photographs show that the included angle of the spray decreases with increasing axial distance from the injector, particularly at high chamber pressures. Strong entrainment of liquid droplets toward the spray axis is already visible within the first 5 mm from the nozzle. Asymmetry was also observed. Droplet sizing was completed at 1600 IPM, 0.1 MPa chamber pressure, a nominal fuel flow rate of 20 mm3 per injection, an axial distance 25 mm from the nozzle, and 1 ms after the poppet opened. Sizes of 1.9 microns diameter and larger were measurable using the Malvern Particle Sizer. The maximum diameter was 11 microns and the Sauter Mean Diameter was approximately 2.7 microns. Thus, the spray from this injector is considerably finer than those from similar poppet injectors without air-assist.