Thermochemical storage with moist air is a promising way to achieve seasonal storage of solar heat. This paper investigates the link between outlet and inlet airflow conditions in the case of hydration/dehydration of thermochemical processes operating with moist air at atmospheric pressure. A thermodynamic analysis of a solid/gas reactor operating in open mode is performed. That leads to a thermodynamic relation called charge–discharge operating line. Experimental tests have been carried out with two different reactive salts in order to prove the validity of the charge–discharge operating line. Experimental results provide a close match between the theoretical approach of the charge–discharge operating line and the measurements. Based on this relation, a reactor performance criterion, called reaction effectiveness, is defined. Both operating line and effectiveness stand as efficient design tools to predict performances of the reactor and to guide R&D choices to enhance them.