Solar air conditioning units present a promising solution for air conditioning, especially desiccant units, in terms of environmental protection and energy saving. In this context, the aim of this paper is a presentation of a new solid desiccant solar air conditioning unit for office spaces in Tunisia and a simulation study of its functioning. Thus, mathematical models for each component are developed which are based mainly on thermal and mass balances. Three mode of functioning are simulated for three climate cases: relatively cold and humid for Bizerte, hot and dry for Remada and finally moderate for Djerba. The results show that the conditioned air obtained in each mode of functioning can ensure comfortable conditions for the office space occupants.