An advanced gaseous tracer technique and procedures were developed and executed to study for the first time the axial dispersion of the gas phase in a slurry bubble column reactor (SBCR) using air-C9 C11 -FT catalyst. Residence time distribution (RTD) curves were obtained by measuring the pulse-input's response of the gaseous tracer. The gas phase axial dispersion coefficient (Dg ) was obtained from minimum square error fit of the one-dimensional axial dispersion model to the measured tracer response data. The effects of solids loading on the axial dispersion of gas phase and the overall gas holdup have been studied. It was demonstrated that increasing solids loading improves the gas axial dispersion while decreasing the overall gas holdup. This work suggests that gas phase axial dispersion is significant in reactor performance evaluation of bubble columns or slurry bubble columns.