The vapor of the Na-Sn-Br-I system was investigated under equilibrium conditions in the temperature range between 623 and 913 K by the use of Knudsen effusion mass spectrometry with a two-compartment Knudsen cell. The gaseous species NaBr, NaI, (NaBr)2, Na2BrI, (NaI)2, SnBr2, SnBrI, SnI2, NaSnBr3, NaSnBr2I, NaSnBrI2, and NaSnI3 were detected in the vapor. The enthalpy and entropy changes of the reactions NaSnBr2I(g) = NaI(g) + SnBr2(g) [1], NaSnBrI2(g) = NaBr(g)+ SnI2(g) [2], and SnI2(g) + 2NaBr(g) = SnBr2(g) + 2NaI(g) [3] resulted as DELTAH-degrees-298 = 172.9 + 6.4 kJ mol-1, DELTAS-degrees-298 = 140.5 +/- 9.9 J mol-1 K-1 for reaction Eq. 1, DELTAH-degree-298 = 181.6 + 5.8 kJ mol-1, DELTAS-degrees-298 = 143.1 +/- 9.3 J mol-1 K-1 for reaction Eq. 2, and DELTAH-degrees-298 = -12.9 +/- 1.7 kJ mol-1, DELTAS-degrees-298 = 3.9 +/- 3.5 J mol-1 K-1 for reaction Eq. 3. Gas-phase equilibria important for the development of metal halide lamps were computed using the results of this work and ancillary data.