The design-point dehumidification performance (i.e., at ARI conditions) of a rotary dehumidifier wheel depends on its rotational speed, the sorption properties of the desiccant, the heat and mass transfer characteristics of the matrix, and the size of the dehumidifier. However, the real operating conditions of a rotary dehumidifier can vary significantly from the design point, given the large variations in operating conditions (i.e., the outdoor, indoor, and regeneration temperatures and humidities)for various locations during different times of the year. This paper investigates the variability of the dehumidification performance of a rotary dehumidifier and its dependence on operating conditions. Also, the effect of the operating conditions on the optimum rotational speed of a rotary dehumidifier where the performance of a rotary dehumidifier is optimized, is described.