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The mechanism of stress corrosion cracking in pure iron base alloys was studied in terms of the effect of the carbide forming nature of a series of addition elements upon the location within the material of the interstitial elements contained in the alloys. The iron used was of very high purity, in order that the variables to be assessed were uninfluenced by extraneous composition effects. Susceptibility to s.c.c. in an aqueous calcium/ammonium nitrate solution at 110+/-C was found to be dependent upon the carbide forming strength of the addition element. The transitional s.c.c. behaviour of an Fe-1%Mn alloy was used to assess the activation energies involved in obtaining immune and susceptible behaviour. A study was made of the nature of time-to-failure values, and it was found that immune and susceptible materials behaved similarly potentiometrically, with susceptible materials failing at an early point in the established pattern. A theory of the mechanism of failure has been suggested that involves electrochemical attack, an adsorption process and brittle fracture. The susceptible behaviour of alloys was correlated with the restricted slip in the grain boundary regions brought about by the presence there of interstitial elements. Suggestions have been made for further research.