A new suspended-combustion-test method involving high-speed digital microscopy and thermal measurements was developed to analyze small tablets of CuS, FeS, and CuFeS2 to elucidate their combustion behavior and phase changes in flash smelting. Moreover, the frequency of splash generation and homogeneous oxidation of the released gaseous sulfur from each sulfide were monitored. Microstructural analysis of quenched samples by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy revealed that the combustion intensity could be related to the formation of an oxide layer, which blocked the O2 supply to the sample interior. Moreover, the gas formed at various oxygen concentrations agitated the samples. In particular, the generated solid-oxide shell could move, the sulfide was exposed to the surface again, and the combustion reaction proceeded further. Insufficient agitation at lower O2 concentrations resulted in a thick and dense oxide layer, which blocked the O2 supply and caused extinction.