Mixtures of polyethylene(PE) and polyvinylchloride(PVC). materials which are similar in structure but different in carbonization behavior. were carbonized in a sealed tube under pressure of 30 h4Pa. The carbonization process under pressure and the morphology of carbon obtained were studied. Elementary carbon in the starting materials converted into the gaseous hydrocarbons. the Raky carbon which is deposiled on the side walls of the gold tube and the carbon which formed at the bottom by liquid-phase carbonization. A remarkable increase in the yield of the bottom carbon was found when a small amount of PVC is added to PE. In the range of PVC content of i-20%. separate carbon spherules of rather uniform size I-_._ 3 5 pm were obtained. Above 2094 the relative amount of aggregates of spherules increased with increase in PVC content and a porous coke-like carbon residue was -obtained above 70% PVC. PVC and PE seem to decompose thermally independently, the former giving active carbon chains at relatively low temperature of 250Β°C and the latter pitch-like viscous liquid above 540Β°C. The resulting two carbonaceous residues interact with each other to give separate carbon spherules when a small amount of active carbon chains and large amount of pitch-like residue are present.