Investigation of the feasibility of the thermochemical two-step water splitting cycle based on MnFe2O4/Na2CO3 system is reported. influence of temperature and carbon dioxide pressure on the oxygen-releasing step was investigated. XRD analysis was applied to obtain phase identification of reacted powders at investigated experimental conditions. Different sodium sub-stoichiometric Na1-delta(Mn1/3Fe2/3)O2-delta/2 compounds were observed and their structure determined by using Rietveld analysis. Selected experimental conditions permitted to define a T/p(CO2), phase diagram, showing different solid phases coexistence regions. Experimental conditions that permit complete regeneration of the initial MnFe2O4/Na2CO3 mixture were identified (field I in the reported diagram), demonstrating the possibility of full chemical cyclical operation of the system. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.