The early stage of electrochemically-induced Fe(001) passivation is investigated via a sub-second resolution real-time X-ray reflectivity technique. Static measurements clearly demonstrate that the passive layer is composed of two distinct layers comprising a dense inner oxide layer and a loosely packed outer layer. The growth of the inner passive layer is mostly completed within 1 s, while the growth rate in the later process is much slower and is qualitatively consistent with typical growth models such as the high-field model and point defect model. The rate-determining mechanism in the early stage is clearly different from that in the later stage. (C) 2019 The Electrochemical Society.