The pyrolysis products of polymethylsiloxanes (PMS) have been studied in order to characterize their phase composition from the viewpoint of equilibrium thermodynamics. Dilatometry measurements and redox analyses with PbO have been conducted and the results analysed and compared with respect to the second law of thermodynamics. In agreement with equilibrium thermodynamics, which shows that there is no ternary compound in the Si-O-C system, the experimental results imply that the pyrolysis residue is an amorphous or nanocrystalline composite consisting of a SiO2-phase matrix with embedded SiC and C particles. These findings seem to contradict the accepted scientific opinion of ternary silicon-oxycarbide phases.