A preliminary investigation of the defect structure of the monoclinic zeta phase, within the interfacial region of Fe-Zn couples, has been performed using cross-section transmission electron microscopy (TEM). Twin boundaries and dislocations have been unambiguously identified, however, examples of defects which, as of yet are unknown, are also presented. The monoclinic zeta phase was found to twin by a rotation of 180° about the normal to the (110) plane.