Monohydrocalcite (CaCO3·H2O) is a hydrated calcium carbonate that has been discovered in various natural settings, including lacustrine environments, cold springs, speleothems, and marine environments. This study investigates fifteen data sets of a monohydrocalcite sample using synchrotron powder X-ray diffraction (XRD) at varying temperatures. Monohydrocalcite, a single phase mineral, transforms into calcite (CaCO3) as the temperature increases. The results show that pure monohydrocalcite exists within the temperature range from 50 to 148 °C in Stage 1, both monohydrocalcite and calcite coexist from 159.5 to 194.5 °C in Stage 2, while pure calcite exists beyond 194.5 °C in Stage 3. When monohydrocalcite is heated from room temperature up to 205 °C, distinct unit-cell parameters are obtained from 159.5 to 194.5 °C for both monohydrocalcite and calcite in Stage 2. Thereafter, calcite is the dominant phase. When the temperature is greater than 194.5 °C, monohydrocalcite entirely disappears; the dehydration process is completed, leaving only calcite. The unit-cell parameters a, c, and V of monohydrocalcite and calcite change with temperature. In Stage 1, the unit-cell parameters a, c, and V of monohydrocalcite increase with temperature ranging from 50 to 148 °C. In Stage 2, as the temperature increases from 159.5 to 194.5 °C, the unit-cell parameters of monohydrocalcite coexisting with calcite increase, while dehydration takes place simultaneously. In Stage 3, the a parameter of calcite decreases with temperature increasing from 194.5 to 205 °C, while the c parameter continues to increase, and V decreases slightly; dehydration is completed at 194.5 °C. In Stage 1, the average and the average bond distances of monohydrocalcite increase with temperature from 50 to 148 °C. In Stage 2, the average bond distance of monohydrocalcite coexisting with calcite also increases with temperature from iii 159.5 to 194.5 °C. The average bond distance changes gradually; the average bond distance decreases, and the average bond distance of calcite increases with temperature from 159.5 to 194.5 °C. Dehydration leads to the disappearance of the O-H bond and the rearrangement of Ca–Oc. In Stage 3, the average bond distance of calcite increases due to thermal expansion from 194.5 to 205 °C. The average bond distance in calcite decreases as the temperature rises from 194.5 to 205 °C. In Stage 1, isotropic displacement parameters UCa, UC, and UO of monohydrocalcite increase from 50 to 148 °C and UOw fluctuates. In Stage 2, UCa and UO of monohydrocalcite coexisting with calcite fluctuate with temperature increasing from 159.5 to 194.5 °C; UC increases in Stage 2, and dehydration leads to the rearrangement of atomic positions. In Stage 3, the UC of calcite decreases with increasing temperature from 194.5 to 205 °C, and UCa and UO increase with temperature.