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The amount of hydrogen released after high-pressure hydrogen loading in carbonate was evaluated. Calcium carbonate (CaCO3) and Mg-substituted CaCO3 were selected as adsorbents. Their crystal structures, specific surface areas (SBET), hydrogen content at high pressure, and the amount of hydrogen released in water were investigated. Calcitic CaCO3 and (Ca0.94,Mg0.06)CO3 structures were identified for the respective samples, and their adsorption forms were found to be of type II as per IUPAC classification. The SBET values were 2.9–6.8 m2/g. The hydrogen content in (Ca0.94,Mg0.06)CO3 was found to be higher than that of calcite CaCO3 at high pressures. Furthermore, the amount of hydrogen released by the (Ca0.94,Mg0.06)CO3 sample was ~20-times higher than that from calcitic CaCO3, measured by gas chromatography. Keywords: Carbonate, Hydrogen absorption/release, Loading hydrogen at high pressure, Gas chromatography (GC)