Caustic cracking is the term used to describe one of the forms in which stress-corrosion cracking manifests itself in carbon steels. In the present study, persistent leakage occurred after ten weeks of service from tube expansions in the steam and mud drum of a two-drum D type boiler, which failed to respond to repeated expansion. The leakage was traced to circumferential cracking in the portion of Fe-0.11C-0.46Mn-0.018S-0.011P tubes within the expanded region. Microscopic examination indicated that all cracks started from the outer surface of the tubes in the expanded portion. The form of cracking which was mostly intergranular. Examination at higher magnification disclosed that a selective attack had taken place on the carbide constituents of the pearlite grains. An alkaline deposit on the fireside surface of the tube resulted from the evaporation of boiler water which had found its way past the tube expansions. This indicated that this operation had not resulted in a satisfactorily tight joint.