The vapor of the NaBr-DyBr3 system was investigated under equilibrium conditions between 763 and 1009 K by using Knudsen effusion mass spectrometry with one- and two-compartment Knudsen cells. The gaseous species NaBr, (NaBr)2, DyBr3, (DyBr3)2, NaDyBr4, and Na2DyBr5 were detected in the vapor. The enthalpy and entropy changes of the reactions NaDyBr4(g) = NaBr(g) + DyBr3(g) [1], NaDyBr4(g) = NaBr(g) + DyBr3(s) [2], and 2NaDyBr4(g) = 2NaBr(g) + (DyBr3)2(g) [3], as well as the enthalpy change of the reaction Na2DyBr5(g) = 2NaBr(g) + DyBr3(g) [4], resulted as DELTAH298-degrees = 242.7 +/- 5.6 kJ mol-1, DELTAS-degrees-298 = 146.5 +/- 6.8 J mol-1 K-1 for reaction Eq. 1; DELTAH-degrees-298 = -33.6 +/- 1.0 kJ mol-1, DELTAS-degrees-298 = -54.9 +/- 2.7 J mol-1 K-1 for reaction Eq. 2; DELTAH-degrees-298 = 269.6 +/- 6.8 kJ mol-1, DELTAS-degrees-298 = 131.8 +/- 8.0 J mol-1 K-1 for reaction Eq. 3; and DELTAH-degrees-298 = 418.8 +/- 12.1 kJ mol-1 for reaction Eq. 4. The following equilibrium constant was obtained for the dissociation reaction Eq. 1 : log K-degrees-p = -12,299/(T/K) + 6.9615. The data are discussed generally as well as with respect to their significance for chemical-vapor transport in metal halide lamps.