The mechanical behavior and microstructural evolution of 17-4 PH stainless steels in three conditions, i.e. unaged (Condition A), peak-aged (H900) and overaged (H1150), exposed at temperatures ranging from 200 to 700degreesC were investigated. The high-temperature yield strength of each condition decreased with an increase in temperature from 200 to 400 C except for Condition A at 400degreesC with a longer hold time where a precipitation-hardening effect occurred. At temperatures from 500-700degreesC, the decrease in after-exposure hardness of Condition A and H900 at longer exposure times was caused by a coarsening effect of copper-rich precipitates. A Similar microstructural change was also responsible for the hardness of H1150 exposed at 700degreesC decreasing with increasing exposure time. Scanning electron microscopy (SEM) observations indicated that the matrix structures of Condition A and H900, when exposed at 600degreesC and above, exhibited lamellar recrystallized alpha-ferrite in the tempered martensite and the size and quantity of these lamellar ferrite phases increased with exposure time. X-ray diffraction (XRD) analyses showed that the reverted austenite phase in H1150 that formed during the over-aging treatment was stable and hardly affected by deformation at temperatures of 200-400degreesC. (C) 2003 Kluwer Academic Publishers.