Indoor air quality depends on the concentration and distribution of airborne contaminants. Often these are not uniform throughout the environment because the inlet air may not mix completely with the room air to dilute contaminants, which can be the result of poor room design or nonequilibrium conditions. The structure of the airflow and the extent of contaminant mixing depends on the location of flow and contaminant sources and sinks, as well as on the geometric barriers in the room. This paper describes a three-dimensional numerical simulation of the transport and dispersion of a passive, airborne contaminant in which the Navier-Stokes and concentration equations are solved under transient conditions for a rectangular enclosure with a single ventilation inlet and outlet. This investigation focuses on the effects of flow obstructions and ventilation rates on contaminant mixing.