The Na + U + O system has been investigated by combined mass-loss and mass-spectrometric Knudsen effusion studies. The dissociation of trisodium uranium(V) oxide (Na 3UO 4) through the vaporization of Na(g) was followed by mass-loss and mass-spectrometric observations. The five three-phase regions traversed by the dissociation of Na 2UO 4(s) were identified by X-ray diffraction analysis. These three-phase regions are: (1) Na + UO 2 + Na 3UO 4, (2) UO 2 + NaUO 3 + Na 3UO 4, (3) NaUO 3 + Na 4UO 5 + Na 3UO 4, (4) NaUO 3 + Na 4UO 5 + Na 2UO 4, and (5) NaUO 3 + Na 2UO 4 + Na 2U 2O 7. Sodium pressures as a function of temperature were measured in each three-phase region; the oxygen pressure was too low for direct measurement except in the last-mentioned three-phase region. From these results and appropriate dissociation reactions, oxygen pressures have been calculated for each of the three-phase regions. The calculated values for the Na + UO 2 + Na 3UO 4 region at 1000 and 1200 K are 3.24 × 10 −35 and 1.62 × 10 −27 atm, respectively. Also, the enthalpies of formation were calculated for five sodium + uranium + oxygen compounds; the values at 1000 K are −(479.0 ± 2.5) kcal th mol −1 (Na 3UO 4), −(554.9 ± 9.5) kcal th mol −1 (Na 4UO 5), −(430.9 ± 6.3) kcal th mol −1 (Na 2UO 4), −(344.2 ± 4.2) kcal th mol −1 (NaUO 3), and −734.8 ± 7.5) kcal th mol −1 (Na 2U 2O 7). The standard enthalpy of formation of Na 3UO 4(s) was found to be ΔH f o (Na 3UO 4, c, 298.15 K) = −(477.8 ± 3.0) kcal th mol −1.