Dehumidification is one of the important process of environment control of buildings in air conditioning systems and an important part of desalination systems. It is necessary to know the energy efficiency of dehumidification processes to determine the maximum efficiency. Exergy analysis is a commonly used technique to identify the improvement potential for systems and processes. This study presents a theoretical analysis for estimating the minimum work requirement in dehumidification processes based on the second law of thermodynamics without considering used devices. The dehumidification process is considered on the saturation line with sensible cooling and isothermal humidifying the humid air gradually. Exergy analysis is separately applied on each process. Depending on the reversible work concept, this approach results in more accurate estimation for minimum work requirement in the dehumidification process. In this paper the variations of the minimum work requirement for the air conditioning and desalination processes, with the ambient conditions, are calculated and evaluations of the effects of variations of the final conditions on minimum work requirement is conducted.