Relations between the evolution peaks and the age-hardening in 18%Ni-Co-Mo maraging steels have been investigated by means of calorimetric analysis and hardness testing. The specific heat versus temperature curve of the quenched specimen is characterized by the appearance of three evolution peaks at 350, 470 and 530°C, respectively, and also an absorption peak at about 500°C. The 470°C peak which seems to be caused by the formation of Mo zones, is clearly observed in alloys containing the critical amount of coexisting Co and Mo. After various age-hardening treatments, the behavior of heat evolution during reheating have been compared with those of the hardness tests. The results are useful in providing information on the aging reaction of the 470°C peak and its relation to that of the 530°C peak. In 475°C isothermal aging, for example, Mo zones are formed in the early stage of hardening, and then these zones are considered to be replaced by the intermetallic compounds in the later stage which corresponds to the aging reaction of the 530°C peak.