The standard enthalpy of formation of perspective solid electrolyte Li7La3Zr2O12 is determined using thermodynamic cycles based on the measurement of its enthalpy of dissolution. The standard molar enthalpies of dissolution in aqueous solution of 1moldm−3 HCl, formation from binary oxides and formation from elements for tetragonal form of Li7La3Zr2O12 are found to be −1547.6, −168.0 and −7152.6kJmol−1, respectively.