An experiment was carried out to investigate the buoyancy induced spatial and temporal flow transition and the associated heat transfer processes in a mixed convective air flow through a bottom heated horizontal rectangular duct, by measuring the local heat transfer coefficient, spanwise temperature distributions, air temperature variation with time at selected locations and by visualizing the cross plane secondary flow for the Reynolds numbers ranging from 9 to 186 and the Grashof numbers up to 5 x 10(6). The results indicated that the heat transfer enhancement is due to the formation and development of a buoyancy driven secondary vortex flow. The onset of thermal instability was found to move upstream for a higher Grashof number and to be delayed for a larger Reynolds number. At increasing Grashof numbers the measured spanwise time-averaged fluid temperature distributions and the corresponding instantaneous flow visualizations clearly show the formation of secondary vortex flow and the changes in the flow structures. At slightly supercritical Grashof numbers, the secondary flow is in the form of two pairs of longitudinal rolls with the vortex flow ascending along the side walls. For higher Grashof numbers the vortex rolls rotate in opposite directions, with the secondary flow descending near the side walls. At even higher Grashof numbers, merging of the vortices takes place and only one pair of vortex rolls exist. Furthermore, raising the Grashof. number or lowering the Reynolds number causes the flow to change from a laminar time periodic to a transitional quasiperiodic and even to a chaotic turbulent state. Based on the present data, a regime map delineating the temporal state of the flow and a correlating equation for the Hopf bifurcation were proposed.