The enthalpy increments of Y2Si2O7 and Dy2Si2O7 have been measured by drop-calorimetry from T = (516.7 to 928.4) K, and T = (516.7 to 918.2) K, respectively. The standard molar enthalpy of formation of monoclinic Y2Si2O7 has been deduced from its enthalpy of solution in (6.20 mol.dm(-3) HCl + 0.147 mol.dm(-3) HF)(aq), and the value Delta(f)H(m)(o)(298.15 K) = -(3854.8 +/- 6.8) kJ.mol(-1) has been obtained. For Dy2Si2O7 the standard molar enthalpy of formation -(3776.1 +/- 4.6) kJ.mol(-1) has been estimated.