Several kinds of hydrated sodium bronze with various amount of hydrogens were synthesized and the standard enthalpies of formation were determined by solution calorimetry. These enthalpies were summarized by least square method and were compared with the enthalpies of formation of hydrogen bronzes. It was found that the enthalpy of formation of the hydrated sodium bronzes was smaller than that of the hydrogen bronze and decreased with the increase in hydrogen content. (C) 2003 Elsevier Science B.V. All rights reserved.