High-temperature combustion techniques have recently attracted interest with regard to the improvement of the thermal efficiency of combustion systems. Fuel pyrolysis is an important factor, as it can affect such flame structures at high temperatures. In this study, the pyrolysis of methane, propane, and dimethyl ether (DME) was measured and the results were compared with theoretical predictions. Pyrolyzed fuels were quenched to room temperature before being introduced onto the burner. Thus, the pyrolysis effects on laminar non-premixed jet flames could be distinguished from many other complex thermal effects. It was found that the flame length was not notably extended in spite of the great increase in the volumetric flow rates resulting from the pyrolysis. In contrast, fuel pyrolysis could significantly affect the soot formation process,and the number of smoke points could be sharply reduced depending on the pyrolysis temperature. Distributions of the luminous intensity and scattering intensity levels in the soot region were discussed in terms of the soot temperatures obtained with a two-color method. Although the adiabatic flame temperatures of the pyrolyzed fuels were theoretically increased, the actual soot temperatures could be reduced when the soot particles were excessively formulated, as in the case with propane flames.