Microstructural development and aging reactions in a commercial 17-4 PH (precipitation hardening) stainless steel have been investigated. It is found that after solution treatment, the steel produces a uniform matrix, which consists chiefly of lath martensite with micro-twins. The age-hardening behavior in the steel studied is similar to that in classical age-hardening alloys. The tempering of martensite has little effect on hardness during aging process in this steel. Transmission electron microscopy (TEM) has revealed the copper precipitates in the specimen aged at 480 degreesC for I h, i.e. a peak aging condition. However, at a higher aging temperature, 620 degreesC, which is much lower than the A(C1) temperature, in addition to the formation of copper precipitates, the reformation of austenite has been noticed. The possible reason why the phenomenon of reversed austenite takes place has been discussed. (C) 2002 Elsevier Science B.V. All rights reserved.