Flourine bomb calorimetric measurements of the massic energy of the reaction: Si3N4(cr,alpha- or beta-) + 6F(2)(g) = 3SiF(4)(g) + 2N(2)(g) have shown that, within the combined uncertainties of the experiments, there is a negligible enthalpy difference between the alpha- and beta-forms of Si3N4 at the temperature 298.15 K. The standard molar enthalpies of formation at T = 298.15 K have been determined: Delta(f)H(m)(o),(alpha-Si3N4) = -(828.9 +/- 3.4) kJ mol(-1), Delta(f)H(m)(o)(beta-Si3N4)=(827.8 +/- 2.5) KJ.mol(-1). These results are approximately 40 kJ.mol(-1) more negative than the recommended value given in the most recent authoritative compilations of chemical thermodynamic information for inorganic substances. The enthalpy difference between the alpha- and beta-forms, (1 +/-4) kJ.mol(-1) at T = 298.15 K, is too small to be measured reliably by combustion calorimetry, but it supports an earlier estimate of 1.25 kJ.mol(-1) from a modeling study.