Experiments on the effects of thermally cracked components of n-dodecane on the ignition and the combustion performance of a scram jet model combustor at Mach 2 were performed. Gaseous components of the thermally cracked n-dodecane at various temperature and 1MPa were measured by a gas chromatograph. Surrogate mixtures including hydrogen, methane, ethane and ethylene were injected into the supersonic air stream in the model combustor, and ignition and combustion behaviors were investigated at total temperatures of 1800 to 2400K. Pressure measurements on the combustor wall and optical observations of combustion using a high speed video camera were conducted. Results indicated that three combustion modes of intensive, transient and weak combustions were observed. Ignition was observed in the boundary layer positioned at the downstream of the cavity, and the flame propagated upward to the cavity. The ignition performance of ethylene was superior to methane and ethane, and their performances were elucidated from the reactivities predicted by a 0-dimensional calculation of chemical reactions for stoichiometric mixtures. An increase in the methane concentration reduced the ignition performance, and an increase in ethylene enhanced the ignition and combustion.