A temperature and humidity controller is designed for a direct expansion air conditioning (DX A/C) system making use of a state feedback decoupling approach of nonlinear control systems. It is shown that the nonlinear dynamics of a DX A/C system can be input-output decoupled by dynamic state feedback. The resulting decoupled system is of minimum phase. Thereafter, the decoupled model is used to design a pole placement controller with guaranteed stability. Unlike controllers based on approximate local linearization of the DX A/C model, the controller proposed is global in the sense that it can track temperature and humidity setpoints in the complete operating range of the DX A/C system. Effectiveness of the controller designed is demonstrated by simulation results.