The K2CO3–CaCO3 system is important both in materials science as a source of new nonlinear optical materials and in the Earth science as a sub-system modeling phase relations in fluxing component of mantle rocks responsible for the generation of deep-seated magmas. Existing data on phase relations in the K2CO3–CaCO3 system at ≤ 0.1 and 6 GPa show significant difference in intermediate compounds and, therefore, do not allow any interpolation between these pressures. Here, we report experimental results on melting and subsolidus phase relations in the system K2CO3–CaCO3 at 3 GPa and 800–1285 °C. At 800 °C, the system has two intermediate compounds: K2Ca(CO3)2, synthetic analog of mineral buetschliite, and K2Ca2(CO3)3. As temperature increases to 850 °C, a third intermediate compound, K2Ca3(CO3)4, appears. The calcite–aragonite transition boundary is located at 962 ± 12 °C. Maximum solid solution of CaCO3 in K2CO3 is 18 mol% at 950 °C. The K carbonate–K2Ca(CO3)2 eutectic is established near 970 °C and 56 mol% K2CO3. The melting point of K2CO3 corresponds to 1275 ± 25 °C. K2Ca(CO3)2 melts incongruently at 988 ± 12 °C to produce K2Ca2(CO3)3 and a liquid containing 53 mol% K2CO3. K2Ca2(CO3)3 melts congruently just above 1100 °C. The K2Ca2(CO3)3–K2Ca3(CO3)4 eutectic is situated near 1085 °C and 29 mol% K2CO3. K2Ca3(CO3)4 melts incongruently at 1100 °C to produce calcite and a liquid containing 28 mol% K2CO3. Considering our present results and previous data on the K2CO3–CaCO3 system, a range of K-Ca double carbonates changes upon pressure and temperature increase in the following sequence: K2Ca(CO3)2 (buetschliite), K2Ca2(CO3)3 (≤ 0.1 GPa; < 547 °C) → K2Ca(CO3)2 (fairchildite), K2Ca2(CO3)3 (≤ 0.1 GPa; 547–835 °C) → K2Ca(CO3)2 (buetschliite), K2Ca2(CO3)3, K2Ca3(CO3)4 (ordered) (3 GPa; 800–1100 °C) → K8Ca3(CO3)7, K2Ca(CO3)2 (buetschliite), K2Ca3(CO3)4 (disordered) (6 GPa; 900–1300 °C).