Desiccant/evaporative cooling systems offer an environmentally benign alternative to conventional vapour compression chillers. A desiccant wheel is the heart of this heat driven cooling system and it uses a solid desiccant for dehumidification with silica gel being the most widely used. Proper design of the wheel is important for the successful operation of the system and theoretical models are useful tools in predicting the performance and optimizing the design. In this paper, two heat and mass transfer models of a counter flow desiccant wheel, one considering only the gas-side resistance, and the other considering both solid-side and gas-side resistances are developed. The models show good agreement with experimental data. The model is used to conduct a comparative study on the performance of different wheel designs. The study shows that the introduction of an axial cooling section can improve the performance of the wheel considerably.