Both cooling fins and closed two-phase thermosyphons are widely used in many fields for passive thermal management. Assuming a favorable gravitational field, the development of a cooling fin thermosyphon device could enhance the heat transfer capabilities of a cooling fin alone. A particular application of interest is that of an air-cooled cooling fin thermosyphon with a large condenser section. The successful development of this type of device could eventually be utilized in a broad range of air-cooled cooling fin applications. An airfoil-shaped thermosyphon design was developed to minimize drag in a cool- ing airflow. The thermosyphon material is copper, and the working fluid used is deionized water. The effect of cavity shape and fill volume on performance (rate of heat transfer) was investigated. The two cavity shapes tested were cylindrical and slot-shaped, and the three fill volumes tested were 0%, 5%, and 20% (as a percentage of the entire cavity volume). The condenser section was air-cooled in a wind tunnel at a wind speed of 100 mph and subjected to evaporator temperatures ranging from 250 F to 325 F. The measured rate of heat transfer is highest for both the slot-shaped and cylindrical cavity thermosyphons at 5% fill volume. The heat transfer rate for the slot-shaped cavity thermosyphon is over 100% larger at 5% fill volume than it is at 0% fill volume. This dramatic increase in performance upon the addition of a small amount of working fluid for the slot-shaped cavity thermosyphon is indicative of the positive effect that thermosyphon technology has on the performance of a cooling fin alone. For the cylindrical cavity thermosyphon, the percent increase between the 0% and 5% fill volumes is approximately 25%. For both thermosyphons, the heat transfer rate increases with evaporator temperature for all fill volumes.