Solar desalination requires an efficient method of evaporation and condensation at relatively low temperatures. This could be suitably achieved in a humidification/dehumidification process using circulated air to enhance evaporation of the water. With proper utilization of the latent heat of condensation of water vapor, the process efficiency could be made high. In order to optimize the design of such a desalination unit, a simulation program was constructed in which the set of non-linear equations describing the desalination unit were solved numerically. The results of the simulation were in agreement with the experimental results of two different units constructed in Jordan and Malaysia. The performance of each unit was tested when they were operated with an energy obtained either from solar or an electrical heating source. The air flow rate was found to have an insignificant effect on the productivity of desalinated water. The simulation allows the proper choice of the feed water flow rate to the unit.