Samples from a used automotive tire (natural rubber based) were subjected to thermal decomposition in a flow reactor at various temperatures with water continuously introduced at various rates. Pyrolysis experiments were also carried out with the samples. It was found, in the wet thermal decomposition, that H2 and CO were the prime components of the gaseous mixture produced, and aliphatic hydrocarbons and alkylbenzenes were the major components of oil product. Also, the surface area of carbon residue was found to increase and the amount of the residue to decrease as water feed and reaction temperature increased. At 1173 K, for example, an 87 m2/g of carbon residue was obtained from pyrolysis, but a 1260 m2/g of carbon residue was prepared from a wet thermal decomposition. As far as a benzene-vapor adsorption test is concerned, this carbon residue is considered to be a good activated carbon.