Phase separation processes like nucleation, crystallization and degradation of a polymer-derived amorphous silicon carbide precursor were quantified by means of thermoanalytical methods (DTA/TG). It is shown that the crystal size of the nanostructure could be controlled and limited by nuclei-inducing heat treatments. Furthermore, the justification for application of the JMAK theory was partially given. A comparison to amorphous SiC processed by other means (ion implanted) was drawn and reveals surprising similarities.