Ferritic stainless steels containing intermediate to h igh interstitial contents (greater than 200 wppm total C + N ) exhibit severe embritt!ement after heating to 950°C and cooling rapidly to room temperature [1, 2]. The underlying cause in type 446 stainless steel is the rapid precipitation of Cr carbides and nitrides at grain boundaries and dislocation sites during cooling from temperatures above 950 °C, where the interstitial atoms are in solution [2]. The complex nature of carbonitride precipitation in Fe-Cr alloys has been the subject of several previous studies of 475 °C embrittlement reaction [1, 3 6]; formation of Cr2N was reported within the first few hours of ageing [3, 4], Fe and C may substitute partially for Cr and N in Cr2N (M2X) [4], and the metastable M2X precipitates coarsen with prolonged ageing and transition to Cr23X6 occurs [5]. Spear et al. proposed that the sequence of interstitial precipitation processes in FeCrA1 alloys is MX--~ M2X---~,M23X 6 [1]. In this work, interstitial preCipitation in a Fe25Cr5A1 alloy has been indentified during ageing at 600°C and thermal cycling treatment processes. The ingot of Fe25Cr5A1 alloy was prepared by induction melting. After homogenizing, the alloy was rolled into rod 8 mm in diameter with intermediate anneals. The altoy chemical composition was (wt %) Cr 24.61, A1 4.91, Mn 0.11, Si 0.35, C 0.038, N 0.007, S 0.002, P 0.015, Ti 0.3, mixed rare earth 0.05, and Fe balance. All alloy specimens were cut from the rod and were solution heated at 1200 °C in air before quenching in water. The programme of thermai cycling treatment used involved heating in air for 3 min at 1200 °C and cooling in air at a rate of 2.8 °C/s for a period which allowed all specimens to cool to ambient temperature. A small amount of residue extracted from the specimens was examined by X-ray diffraction. Thin foils of the alloy specimens for transmission electron microscopy (TEM) studies were prepared by standard procedures and the foils examined in a Philips 400 TEM with an electron accelerating voltage Of 100kV and an incident electron probe diameter of approximately 40 nm; energy-dipersive X,ray spectroscopy (EDS) was used to enable X-ray microanalysis of the specimens. TEM investigation indicates that there is no precipitation at grain boundaries in the alloy quenched from 1200 °C as shown in Fig. 1. A small amount of intragranular blocky TiN is present in the alloy. The matrix of the alloy has a bcc structure (a-