This present work has theoretically investigated coupled heat and mass exchangemechanisms in counterflow rotary dehumidifier with the mixed desiccant of LiCl and additives,used in air conditioning systems. Mathematical model has been derived, based on one-dimensionaltransfer model and Polyni potential theory (theory of water chemical potential)and solved, using digital computer. The profiles of temperature and absolute humidity distributionin the matrix have been obtained. Analysis shows, that the direction of mass transfer isopposite to positive water vapour partial pressure. The results are presented in the form of theoverall effectiveness of the heat exchanger. Dimensionless operating factors, affecting theefficiency of the heat exchanger have been investigated. The received results offer scope forestimation of variation range of rotary dehumidifier optimum operating conditions and suitableclimatic zones for this unit.