Accurate evaluation of vehicles' transient total power requirement helps achieving further improvements in vehicle fuel efficiency. When operated, the air-conditioning (A/C) system is the largest auxiliary load on a vehicle, therefore accurate evaluation of the load it places on the vehicle's engine and/or energy storage system is especially important. Vehicle simulation models, such as "Autonomie," have been used by OEMs to evaluate vehicles' energy performance. However, the load from the A/C system on the engine or on the energy storage system has not always been modeled in sufficient detail. A transient A/C simulation tool incorporated into vehicle simulation models would also provide a tool for developing more efficient A/C systems through a thorough consideration of the transient A/C system performance. The dynamic system simulation software MATLAB/Simulink® is frequently used by vehicle controls engineers to develop new and more efficient vehicle energy system controls. A MATLAB/Simulink-based transient A/C system simulation model is easier to incorporate into MATLAB/Simulink-based vehicle simulation software; therefore, the availability of a transient A/C system simulation tool developed in the MATLAB/Simulink platform is important.NREL has recently developed an A/C simulation tool to address these needs. This paper describes in detail the modeling methods used for this new simulation tool. Comparison with measured data is provided to demonstrate the validity of the model. The agreement between simulation and measurement was shown to be good on both the component and system level. The capabilities of the model are also demonstrated by the example of simulating the SC03 cycle.