Desiccant evaporative cooling system is a potential environment-friendly 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 most widely used. The desiccant material is coated, impregnated or formed in a place on the supporting rotor structure. The matrix consists of multiple channels in the direction of axis of rotation of the wheel. Proper design of the wheel is very important for the successful operation of the system and theoretical models are very useful tool in predicting the performance and design optimization. One of the major difficulties in numerical modelling of the desiccant wheel is the lack of knowledge of adsorption equilibrium of porous medium.In this paper, adsorption equilibria of the silica gel is studied and the different theories existing to characterize the adsorption process and different equations for prediction of adsorption equilibria are compared and used in a theoretical model predicting the wheel performance. The results of the different models are compared with experimental data and conclusions are drawn.