The Standard molar Gibbs energies of formation of Sr3Zr2O7(s) have been determined for the first time by measuring Sr(g) pressures over the three phase {x(1)Sr(4)Zr(3)O(10)(S) + x(2)Sr(3)Zr(2)O(7)(S) + (1-x(1)-x(2))Zr1-xOx(s)} using a Ynudsen effusion mass-loss technique employing a Sartorius micro-balance model 4410. The values of the mole fractions x(1) and x(2) in the three phase mixture are 0.3795 and 0.5391, respectively. The Delta(f)G(m)(0)(Sr3Zr2O7,s, T) has been calculated and is given by:Delta(f)G(m)(0)(Sr3Zr2O7,s, T)/(kJ . mol(-1)) = {-4229 + 0.7357(T/K)} +/- 15, where (1349 K less than or equal to T/K less than or equal to 1466).The Delta(f)H(m)(0)(Sr3Zr2O7,s,298.15 K) value has been calculated from the Delta(f)G(m)(0)(Sr-3 Zr2O7,s,T), and molar heat capacities of Sr(s,1), Zr(s), and O-2(g) from the literature using the values of Sr4Zr3O10(S) from our previous work. The resulting value is - (4186 +/- 38) kJ . mol(-1). A corresponding value of Delta(f)H(m)(0)(Sr3Zr2O7, s, 298.15 K) = - (4163 +/- 42) kJ . mol(-1) has also been calculated using the third law. In this method, Phi(m)(0) = (Delta(0)(T)S(m)(0) - Delta(298.15K)(T) H-m(0)/T) values for Sr4Zr3O10(S), Sr(g), Zr0.72O0.26(S), Sr3Zr2 07(s), and Sr(g) pressure over the three phase system have been used for the calculation of Delta(f)H(m)(0) (Sr3Zr2O7, s, 298.15 K) at each experimental temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.