Abstract: An air cycle air conditioning system including a turbine for receiving air and providing expansion thereof, thus lowering the temperature and pressure of the air, an air--air heat exchanger defining a first air flow path for air to be cooled and a second air flow path for air from the turbine at sub-atmospheric pressure to pass therethrough and absorb heat from the air to be cooled, and a compressor, at least partially driven by the turbine, for drawing air at sub-atmospheric pressure through the second flow path for air, characterized by a pre-heater which heats a flow of air, preferably comprising a mixer of ambient air and recirculation air, and directs this mixed flow to the air--air heat exchanger, thereby increasing its temperature and, as a result, the outlet temperature of the secondary air flow, thereby increasing its water vapor absorption capacity.