The pseudomorphic replacement of mineral barite (BaSO4) crystals into barium carbonate was investigated in the present work by using carbonated alkaline hydrothermal fluids. Hydrothermal treatments were carried out over the temperature range from 150 up to 250 degrees C for intervals between 1 and 192 h, with different filling ratios (40-70%), and CO32-/SO42- molar ratios of 1, 5, and 10. The reaction products were characterized by XRD and SEM techniques. The chemical reactivity of mineral barite crystals was markedly limited at temperatures below 200 degrees C, and only a tiny BaCO3 layer on the surface of the original BaSO4 crystal was formed on the crystal treated for 192 h. The rate of the pseudomorphic conversion of BaSO4 into BaCO3, was accelerated by increasing the reaction temperature and the molar ratio CO32-/SO42-. Powder X-ray diffraction results showed that under hydrothermal conditions the replacement of SO42- ions, in barite crystals was completed at 250 degrees C with a molar ratio CO32-/SO42-= 10 for an interval of 192 h, resulting in the Witherite structure. The morphology of the completely converted BaCO3 at 250 degrees C in a Na2CO3 solution for 192 h, showed that the conversion proceed without severe changes of the original shape and dimension of the original crystal, similar to that observed in mineral pseudomorphic replacement process.