Catalytic effects of metallic salts on the combustion of diesel fuel oil have been studied. In the case of organometallic salt, the active species are the metallic oxides resulted from combustion of the salts. The oxides act only on the residual solid carbon produced from the fuel oil combustion. The catalytic activity can be explained with the semiconductor theory just as in the case of the gas phase reaction. The chemical rate constant of the combustion of carbon, the soot from diesel fuel oil, is found to be $k_c=1.1{ imes}10^4;exp$ (-16,600/T) below $800^{circ}K$. By addition of metallic oxides, the rate constant increases remarkably. This work has substantiated the belief that the effect of the metallic salts on the fuel oil combustion can conveniently be studied by checking directly the effect of the corresponding metallic oxide on the soot carbon.