Pulsed electrodeposition (PED) is used to prepare nanocrystalline iron with an average grain size of 19 nm and thermal stability up to 550 K. At 663 K ≤ T ≤ 783 K the kinetics of grain growth, with respect to size and size distribution, is studied in situ by means of real-time synchrotron X-ray diffraction. The Bragg peak line shapes of the large number of diffractograms are analyzed using a Warren/Averbach procedure improved with respect to reliability and efficiency. We observe two regimes of grain growth: at less elevated temperatures grain growth is smooth and moderate up to limiting size values between 50 and 100 nm, depending on temperature. The initially rather narrow width of the size distribution increases slightly, and the activation energy of grain growth, about 100 kJ/mol, corresponds to the literature value for grain boundary self-diffusion in nanocrystalline Fe. At higher temperatures the grains grow first rapidly and then slowly up to limiting values between 200 and 400 nm, depending on te...