This article investigates the leaking of tubes in a water tube boiler used in a palm oil mill. Leakages were discovered in the roof tubes and the rear wall tubes of the boiler. Samples of the punctured tubes were removed for examination. Micro- and macro-examinations were conducted to determine the causes of failure. EDX/SEM (energy dispersive X-ray spectroscopy/ scanning electron miscroscope) was also conducted to examine the composition of the elements of the specimens and a high concentration of sodium was shown. Key outcomes from the investigation are outlined in this article. They indicate that caustic corrosion was the cause of the leakages of the tubes.