There is a need to develop microstructural methods for assessing the time-temperature history of power plant steels. One of the techniques is based on studies of minor-phase composition changes during service exposure. The chemical compositions of carbides and/or nitrides which precipitate during quality heat treatment of high-alloy steels are expected to be very close to equilibrium. However, the service temperatures of these steels are usually significantly lower than the tempering temperatures before service. Any composition changes of the minor-phases during prolonged service exposure depend on the difference between the equilibrium compositions at tempering and service temperatures. The composition changes during service are very slow and take place over a period of many years. (C) 2003 Elsevier Science B.V All rights reserved.