Phase relations in the system Na2CO3-CaCO3 have been studied in the compositional range, X(Na2CO3), from 100 to 10 mol%, at 6.0 GPa and 900–1400 °C. Below 1100 °C, the system has three intermediate compounds: Na4Ca(CO3)3, Na2Ca3(CO3)4, and Na2Ca4(CO3)5. The Na4Ca(CO3)3 and Na2Ca3(CO3)4 compounds melt congruently slightly above 1200 and 1300 °C, respectively. The eutectics were established at 70 and 52 mol% near 1200 °C and at 21 mol% near 1300 °C. The Na2Ca4(CO3)5 compound decomposes to the Na2Ca3(CO3)4 + aragonite assembly at 1100 °C. Maximum solid solution of CaCO3 in Na2CO3 is 6–8 mol% at 1100–1300 °C. Melting of Na2CO3 occurs between 1350 and 1400 °C. Na solubility in aragonite does not exceed the detection limit (<0.5 mol%). Aragonite remains a liquidus phase at 1300 and 1400 °C.