Rapid etching and clear delineation of prior austenite grain boundaries are characteristic of tool steels upon rapid quenching from sufficiently high austenizing temperatures and of hot-work die steels upon arrested quenching. In this study, two tool steels (M2 and M10 types) and a hot-work die steel (H21 type) were heat treated under a variety of austenizing conditions and examined by extraction replica electron microscopy. In each case, the grain-boundary delineation observed under light metallography was associated with the presence of discrete carbide particles. The presence of these carbides at grain boundaries accelerates the etching rates and delineates the boundaries at the lower scale of resolution in light metallography. The various boundary carbides were identified, and the carbide-precipitation kinetics were quantitatively related to interstitial supersaturation in the alloy. An interesting observation was the presence of M2C as grain-boundary carbides and as residual carbides in the high-speed tool steels.