The accuracy of the tracer-gas method depends on the spatial uniformity of tracer/air mixing, which in many cases is unsatisfactory However, information on this critical problem has been scarce, largely due to the practical difficulty in obtaining experimental data. This paper presents a study of the effect of tracer species on tracer mixing, using a time-dependent computational flow dynamics (CFD) method. It was found that for single-zone tracer decay tests, three tracer-gases, namely sulphur hexafluoride, nitrous oxide and carbon dioxide have virtually identical mixing patterns and thus there is no difference between them in terms of airflow measurement results. However, for multi-gas tests, where there is inter-zonal tracer. movement, the tracer-gases with different binary diffusivities exhibit significantly different mixing behaviours. In these situations, the choice of tracer will impact upon the accuracy of airflow measurement. These results support recent experimental findings concerning the significant effect of tracer species on airflow measurement.