Fe containing Na-ion cathode materials are desirable due to their low cost and use of earth abundant raw materials. Here, the electrochemistry of layered NaxFex3+Ti1-x4+O2 is described. It was found that Mossbauer spectra can accurately detect Na vacancies in NaxFex3+Ti1-x4+O2 material from their effects on neighboring Fe3+ ions and this model can be used to analyse materials at different stages of charge or discharge. This Na vacancy model potentially has broader implications and may be useful as a probe for Na-vacancies in iron containing Na-ion cathode materials in general. The oxidation of Fe3+ to Fe4+ during electrochemical cycling is shown to be a major source of hysteresis and capacity fade, while the activation of the Fe2+/Fe3+ and Ti3+/Ti4+ redox couples results in stable cycling performance. Possible reasons for capacity fade and hysteresis are discussed in terms of ex-situ Mossbauer and XRD studies. (C) 2014 The Electrochemical Society. All rights reserved.