The standard entropy and heat of formation of amorphous silicon oxycarbide, Si5C6O2, have been assessed to be 161.1 J K-1 mol(-1) and -1282.3 kJ mol(-1), respectively, based in part on the literature data of the mass-spectrometric vapour pressure of SiO(g) over a Nicalon fibre. Using the so-obtained standard Gibbs energy of Si5C6O2, new phase relations (predominance diagrams) of the Si-C-O system have been computed and graphically shown for the temperature range 300-2400 K, including such condensed phases as C, Si, SIG, Si2O, SiO, Si2O3, SiO2 and Si5C6O2. The thermal behaviour of Nicalon fibres can be quantified thermodynamically by use of the predominance diagrams.