The microstructure of the high carbon-containing cobalt-based superalloy, Co-101, has been studied in the as-cast state and following a variety of heat treatments. In the as-cast state both M7C3 and Mo-rich M23C6 carbides were observed in the interdendritic regions. After thermal exposure at temperatures between 1000 and 1250˚C for 1, 10, and 100 hours, the M7C3 interdendritic carbide network was observed to transform into Mo-lean M23C6 carbide. These changes were rationalised with thermodynamic calculations. The carbide transformation liberated carbide-forming elements that resulted in precipitation of intragranular carbides in the dendritic regions at temperatures below 1150˚C. These carbides in the cast-aged material preferentially formed at the dendrite peripheries early during exposure, leading to wide particle size distributions. Peak hardness in the cast-aged material was attained within the first 10-hours of exposure and softening was observed thereafter. After solution heat treating at 1250˚C for 10 hours, the microstructure of Co-101 comprised an M23C6 interdendritic carbide network and solid solution dendrites supersaturated with carbide-forming elements. Subsequent ageing of this microstructure for 100 hours at 900˚C led to a high number density and narrow particle size distribution of intragranular carbides. The characteristics of these carbides in the solution-aged material resulted in greater hardness, which was retained for longer durations of exposure, than the cast-aged specimens.