Condensed phase mixtures (Ge+Si+Si3N4) and (Si+Si3N4) were evaporated from boron nitride Knudsen cells and the partial pressures of Si3N(g),Si2NSi2,(g), and Si(g) measured with a mass spectrometer at temperatures between 1770 and 2000 K. The thermal functions for Si3N(g) were calculated from theoretical molecular constants, and the enthalpy change for the dissociation reaction : Si3N(g)=Si2N(g)+Si(g) determined. With the revised atomization enthalpy of Si2N(g), Delta(atom)H(m)(0)=1011+/-12 kJ mol(-1) at T=0 K and 1020+/-12 kJ mol(-1) at T=298.15 K, the atomization enthalpy of Si3N(g) was derived as 1298+/-19 kJ mol(-1) at T=0 and 1312+/-19 kJ mol(-1) at T=298.15 K. These values in combination with the enthalpies of formation of Si(g) and N(g) yielded the enthalpies of formation Delta(f)H(m)(0) at T=298.15 K : 352+/-15 kJ mol(-1) for Si2N(g), and 511+/-22 kJ mol(-1) for Si3N(g). Experimental and theoretical bond dissociation energies have been compared and discussed, indicating a very strong bonding of nitrogen to Si-3.