A liquid desiccant air-conditioning system operable in a cooling and dehumidification mode, a heating and humidification mode, and/or a heating and dehumidification mode, the system comprising: a conditioner for treating a first air stream flowing therethrough and provided to a space, said conditioner using a heat transfer fluid and a liquid desiccant to cool and dehumidify the first air stream in the cooling and dehumidification mode, heat and humidify the first air stream in the heating and humidification mode, and heat and dehumidify the first air stream in the heating and dehumidification mode; a regenerator connected to the conditioner such that the liquid desiccant can be circulated between the regenerator and the conditioner, the regenerator causing the liquid desiccant to desorb water vapor to a second air stream in the cooling and dehumidification mode and in the heating and dehumidification mode, and causing the liquid desiccant to absorb water vapor from the second air stream in the heating and humidification mode; a refrigerant system including a compressor and at least one expansion valve (827, 829) processing a refrigerant; a first refrigerant-to-heat transfer fluid heat exchanger connected to the conditioner and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the conditioner; a second refrigerant-to-heat transfer fluid heat exchanger connected to the regenerator and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the regenerator; a heat transfer fluid-to-air heat exchanger for exchanging heat between the heat transfer fluid used in the regenerator and a third air stream when the air-conditioning system is operating in the cooling and dehumidification mode or the heating and humidification mode, said heat transfer fluid-to-air heat exchanger also being connected to the first refrigerant-to-heat transfer fluid heat exchanger for exchanging heat between the heat transfer fluid flowing in first refrigerant-to-heat transfer fluid heat exchanger and the third air stream when the air-conditioning system is operating in the heating and dehumidification mode; and a valve system (814, 815, 832, 838) for selectively controlling flow of heat transfer fluid among the conditioner, the first refrigerant-to-heat transfer fluid heat exchanger, the second refrigerant-to-heat transfer fluid heat exchanger, the heat transfer fluid-to-air heat exchanger, and the regenerator in accordance with a given mode of operation of the air-conditioning system. a conditioner for treating a first air stream flowing therethrough and provided to a space, said conditioner using a heat transfer fluid and a liquid desiccant to cool and dehumidify the first air stream in the cooling and dehumidification mode, heat and humidify the first air stream in the heating and humidification mode, and heat and dehumidify the first air stream in the heating and dehumidification mode; a regenerator connected to the conditioner such that the liquid desiccant can be circulated between the regenerator and the conditioner, the regenerator causing the liquid desiccant to desorb water vapor to a second air stream in the cooling and dehumidification mode and in the heating and dehumidification mode, and causing the liquid desiccant to absorb water vapor from the second air stream in the heating and humidification mode; a refrigerant system including a compressor and at least one expansion valve (827, 829) processing a refrigerant; a first refrigerant-to-heat transfer fluid heat exchanger connected to the conditioner and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the conditioner; a second refrigerant-to-heat transfer fluid heat exchanger connected to the regenerator and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the regenerator; a heat transfer fluid-to-air heat exchanger for exchanging heat between the heat transfer fluid used in the regenerator and a third air stream when the air-conditioning system is operating in the cooling and dehumidification mode or the heating and humidification mode, said heat transfer fluid-to-air heat exchanger also being connected to the first refrigerant-to-heat transfer fluid heat exchanger for exchanging heat between the heat transfer fluid flowing in first refrigerant-to-heat transfer fluid heat exchanger and the third air stream when the air-conditioning system is operating in the heating and dehumidification mode; and a valve system (814, 815, 832, 838) for selectively controlling flow of heat transfer fluid among the conditioner, the first refrigerant-to-heat transfer fluid heat exchanger, the second refrigerant-to-heat transfer fluid heat exchanger, the heat transfer fluid-to-air heat exchanger, and the regenerator in accordance with a given mode of operation of the air-conditioning system.