In this study, the carbide dissolution and formation of biomedical Co-28Cr-6Mo-0.25C-1Si (1Si) and Co-28Cr-6Mo-0.25C-1Mn (1Mn) alloys occurring during solution treatment and aging were investigated. The addition of Si or Mn markedly affected the behavior of precipitates during heat treatment. The alloy with added Si required a longer solution treatment time for complete precipitate dissolution as compared to the alloy with added Mn. Blocky M23C6-type carbide was observed during solution treatment at 1448–1523 K. Starlike precipitates were observed in the 1Si and 1Mn alloys at 1523–1548 K and 1523 K, respectively. The starlike precipitates exhibited two types of morphologies: dense and stripe patterned. The starlike precipitates with dense and stripe-patterned appearances were a -phase and M23C6¬-type carbide + metallic -phase, respectively. An M23C6-type carbide and an -phase were formed during the aging of both the 1Si and the 1Mn alloys. In addition, an intermetallic -phase was detected during the aging of the 1Si alloy at 1373 K.