Afterburning and nonafterburning exhaust plumes were studied experimentally for underexpanded sonic and supersonic conical circular nozzles. The plume structure was visualized using thermal imaging and regular photography. Thermal emission by the plume is mainly dependent on the presence of afterburning. Temperature and reducing power (or unoxidized fuel content) of the exhaust gases, in addition to the nozzle configuration, determine the structure of the plume core, the location where the afterburning is initiated, its size, and intensity. The temperature, through chemical kinetics, determines the presence of afterburning and its initiation location. Its effect is especially critical in marginally afterburning plumes. The fuel content determines the size and intensity of the plume when afterburning occurs. Comparison between single shot and average thermal images of the plume show that afterburning is a highly turbulent combustion process.