The main aim of the paper is to determine a theoretical range of operational conditions for the three chemical reactions of the Na-O-H (sodium-oxygen-hydrogen) thermochemical water splitting cycle by performing a Gibbs free energy (Delta G) analysis. Delta G is a thermodynamic parameter that allows determining if a chemical reaction is able to occur due to its enthalpy (Delta H) and entropy (Delta S) changes under specific conditions of pressure (p) and temperature (T). The methodology consists of implementing in Engineering Equation Solver (EES) software equations related to Delta H and Delta S for the three chemical reactions of the Na-O-H process, considering specific conditions of p and T with proper thermodynamic data to calculate Delta G. The main obtained results are: reaction 1, hydrogen production step, has Delta G < 0 for any values of T and p considered in the study; it can be performed at 100 degrees C under 1 bar; reaction 2, metal separation step, only can proceed under vacuum atmosphere at around 500 degrees C; reaction 3, hydrolysis step, always has negative Gibbs free energy despite pressure and temperature values and it can be carried out under 1 bar at 25 degrees C. Moreover, the methodology described in this paper could be used to investigate operational conditions for thermochemical processes. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.