The standard molar enthalpies of solution of cerium and lanthanum silicates (beta-Ce2Si2O7 and beta-La2Si2O7) in (6.20 mol . dm(-3) HCl + 0.147 mol . dm(-3) HF) (aq) were measured. From these values the standard molar enthalpies of formation have been deduced : Delta(r)H(m) degrees(Ce2Si2O7, cr, beta, 298.15 K) = -(3507.6 +/- 4.5)kJ . mol(-1) and Delta(r)H(m) degrees(La2Si2O7, CT, beta, 298.15 K) = -(3815.7 +/- 4.5) kJ . mol(-1).