The phenomenon of transgranular stress corrosion cracking is observed in buried pipelines operating in contact with diluted solutions under disbonded coatings. It was proposed that this kind of cracking is explained by the evolution and permeation of hydrogen at the crack tip, as observed in nuclear pressurevessel steels and other low-alloy steels. We discuss the mechanism of transgranular stress corrosion cracking. The combined effect of hydrogen permeation and anodic dissolution is experimentally investigated by imposing different levels of cathodic protection: from the free corrosion potential [−750 mV (SCE)] to −1300 mV (SCE). The comparison of the experimental and literature data shows that both anodic dissolution and hydrogen permeation are active and their (synergic) combination gives rise to this particular kind of cracking. Anodic dissolution is active in the crack enclave. At the same time, the cathodic discharge of protons produces hydrogen which enters steel at the crack tip. This phenomenon is intensified for more electronegative potentials. In addition, inside the cracks, the environment becomes more acid than in the bulk and the rates of both anodic and cathodic reactions increase. Hydrogen penetrating into steel increases its brittleness, which depends on the microstructure of the metal and the stress-strain field. This effect was corroborated in our experiments by monitoring the variations of the hardness (brittleness) of steel, observing the formation of internal cracks by optical microscopy, and examining fracture surfaces by scanning electron microscopy.