This paper describes the latest advancement in air cycle environmental control systems (ECS) for commercial aircraft. The concept, which has been designated the “condensing cycle,” has been selected for use in the 777 and MD-12 air conditioning systems. It incorporates features of the high pressure water separation chilled recirculation systems introduced in the 1980's on the Boeing 757 and 767, the A320, and numerous pressurized commuter aircraft. Additional benefits are realized in the areas of cycle efficiency, control simplicity, moisture removal performance, and ram drag reduction through the incorporation of a four-wheel (two-stage turbine) air cycle machine. A detailed description of the cycle is presented in this paper as well as details of the advantages of this approach relative to three-wheel air cycle machine chilled recirculation systems.