The phase diagrams of Na 2 CO 3 and K 2 CO 3 have been determined with multianvil (MA) and diamond anvil cell (DAC) techniques. In MA experiments with heating, γ -Na 2 CO 3 is stable up to 12 GPa and above this pressure transforms to P63 /mcm-phase. At 26 GPa, Na2 CO3 - P63 /mcm transforms to the new phase with a diffraction pattern similar to that of the theoretically predicted Na2 CO3 - P21 /m. On cold compression in DAC experiments, γ -Na2 CO3 is stable up to the maximum pressure reached of 25 GPa. K2 CO3 shows a more complex sequence of phase transitions. Unlike γ Na2 CO3, γ -K2 CO3 has a narrow stability field. At 3 GPa, K2 CO3 presents in the form of the new phase, called K2 CO3 -III, which transforms into another new phase, K2 CO3 -IV, above 9 GPa. In the pressure range of 9–15 GPa, another new phase or the mixture of phases III and IV is observed. The diffraction pattern of K2 CO3 -IV has similarities with that of the theoretically predicted K2 CO3 - P21 /m and most of the diffraction peaks can be indexed with this structure. Water has a dramatic effect on the phase transitions of K2 CO3. Reconstruction of the diffraction pattern of γ -K2 CO3 is observed at pressures of 0.5–3.1 GPa if the DAC is loaded on the air.