The sublimation of terbium and lutetium triiodides was studied by Knudsen effusion mass spectrometry. The temperature dependencies of partial vapor pressures [atm] of the monomer and dimer molecules were determined as: lnp(TbI3) = – (31.74 ± 0.24)× 103/T + (21.43 ± 0.29) for 743–906 K; lnp(Tb2I6) = – (38.66 ± 0.38)× 103/T + (25.63 ± 0.46) for 786–906 K; lnp(LuI3) = – (31.48 ± 0.25)× 103/T + (22.28 ± 0.30) for 721–951 K; and lnp(Lu2I6) = – (36.95 ± 0.37)× 103/T + (25.21 ± 0.43) for 796–950 K. On the basis of a joint analysis of all literature data, the sublimation enthalpies [kJ mol–1] at 298.15 K are recommended as 281 ± 3 (TbI3), 346 ± 30 (Tb2I6), 279 ± 12 (LuI3), and 340 ± 30 (Lu2I6). The standard formation enthalpies [kJ mol–1] of the gaseous species at 298.15 K are –343 ± 4 (TbI3), –902 ± 30 (Tb2I6),–326 ± 12 (LuI3), and –870 ± 30 (Lu2I6).