The soot from residual fuel oil shows similar structure to that of conradson carbon. When X-ray analysis were performed on soot produced in a practical furnace (cenospher) and on conradson carbon, similar values were obtained.With the object of determining whether addition of metal naphthenate would improve combustion of fuel oil less formation of soot, a study on the effect of metal naphthenate on conradson carbon was made. The results are summarised as follows:(1) In general, the addition of metal naphthenate to residual fuel oil has no effect in reducing the formation of conradson carbon. Ferrous naphthenate, however, reduce the formation of conradson carbon.(2) Cuprous and ferrous naphthenate tend to decrease the activation energy and lower the ignition point of residual carbon in combustion. These effects are considered to result from the catalytic action of the metals.(3) The addition of metal naphthenate did not change the crystal structure of conradson carbon. No particular relation was noted between crystal structure of carbon and its combustion rate.(4) From the above results, conclusion was reached in that proper metal naphthenate (Cu, Fe salts) improves the burning qualities of fuel oil soot (Cenospher).