As part of an investigation of carbonate systems under mantle pressures and temperatures, phase relations in the K2CO3–MgCO3 system have been studied at 3 GPa and 800–1300°C. Subsolidus assemblages comprise the stability fields of K2CO3 + K2Mg(CO3)2 and K2Mg(CO3)2 + MgCO3 with the transition boundary near 50 mol% K2CO3 in the system. The K2CO3–K2Mg(CO3)2 eutectic is located at 840°C and 52 mol% K2CO3. The K2CO3 content in the melt coexisting with potassium carbonate increases to 85 mol% as temperature increases to 1050°C. K2CO3 remains solid up to 1250 and melts at 1300°C. K2Mg(CO3)2 melts incongruently at 890°C to produce magnesite and a liquid containing 51 mol% K2CO3. As temperature increases to 1300°C, the K2CO3 content in the liquid coexisting with magnesite decreases to 27 mol%.