We have synthesized two kinds of amorphous carbon products starting from C-60, one by low-temperature oxidation and the other by vacuum-annealing in a sealed tube at 900 degrees C. Our characterization studies indicate that the former has a nominal composition C5O and consists of two amorphous phases one of which has an amorphous C-60 structure, and may perhaps be a C-60 polymer, whereas the other has an amorphous graphitic carbon structure. The product obtained by vacuum-annealing of C-60 has a negligible oxygen content and is single phase with an amorphous graphitic carbon structure. The TG-DTA thermograms of both these materials in oxygen exhibit close similarity with respect to their ignition temperatures and the two-step burning process. However, their heats of combustion differ. The TG-DTA studies in a helium atmosphere followed by X-ray characterization of the residue indicate that although vacuum-amorphized C-60 is stable and remains amorphous up to 1000 degrees C the amorphous product C5O exhibits appreciable volatility perhaps due to oxygen-induced instability. The volatilization proceeds in two steps and the gas phase may contain carbon clusters. Together with volatilization, crystallization of amorphous C5O also occurs.