Underground hydrogen storage (UHS) promises great storing potential for energy produced from renewables. To make UHS a feasible and safe process, fundamental research investigating the integrity of the cement sheath applied in boreholes against hydrogen exposure is essential. In this study, hydrothermal autoclave experiments are conducted to evaluate hydrogen induced changes in the mineralogical phase composition and the microstructure of a class G cement. X-ray diffraction and scanning electron microscopy combined with nitrogen adsorption/desorption experiments are carried out. Only minor mineralogical changes are observed such as the decomposition of monosulphate and the formation of ettringite. Nitrogen adsorption/desorption experiments reveal comparable pore size distributions with minor differences in porosity and surface area. The findings of this study suggest that hydrogen does not substantially affect the phase composition and microstructure of the investigated low permeable class G cement sheath emphasizing the relatively unreactive nature of downhole cements of this type against hydrogen.