This paper reports on an investigation seeking to better understand the influence of Reynolds number and airstream velocity on dehumidifier performance.The investigation follows a classical heat and mass transfer approach, the results of which are checked by use of log mean relationships. They are then confirmed empirically to apply over a broad thermal environmental range.A major problem that had to be overcome was the need to isolate all other parameters which also influence dehumidifier performance. This was necessary to confine the study only to airstream velocity variation across the dehumidifier in context of the air-conditioning system. New findings show that the role of the dehumidifier to simultaneously cool and dehumidify has an important influence on the results. The implementation of these findings can lead to more efficient operation of the total air conditioning system.