This paper presents an air cooled slab system (ACS) specially designed for use in multi-storey steel framed buildings. The system works by passing air through ducts in the trapezoidal hollow section of the steel/concrete composite floor, so the mass of the concrete slab can be used to store both heat and coolth at different parts the diurnal cycle. Since the ACS system uses the thermal capacity of concrete, it has considerable potential for reducing or even eliminating the need for mechanical cooling. This paper pesents a CFD model of the ACS system that was developed to understand the performance of the system under different operating conditions. The model was validated with experimental data.