This study investigates the solubility of hydrogen and its mixtures with methane in brine at underground hydrogen storage conditions. The experiments were conducted using a PVT cell at temperatures of 45, 50, and 55°C, and pressures ranging from 1 to 500 bar, simulating a wide range of reservoir conditions. Two brine samples representative of different reservoirs were tested, along with mixtures of methane and hydrogen (10 mol% H₂ and 50 mol% H₂) and pure hydrogen. The results show that the solubility curves for pure hydrogen are consistent with literature data, while the impact of brine composition and temperature on solubility was minimal. However, the slope of the solubility curve was more pronounced for gas mixtures with lower hydrogen percentages, indicating that the presence of methane enhances the solubility of hydrogen. The study provides valuable data for estimating the volume of hydrogen that can dissolve in formation water during storage, which is crucial for assessing potential storage losses and the availability of hydrogen for microbial processes. The findings are relevant for the safe and efficient storage of hydrogen in underground reservoirs, particularly in depleted gas reservoirs.This study investigates the solubility of hydrogen and its mixtures with methane in brine at underground hydrogen storage conditions. The experiments were conducted using a PVT cell at temperatures of 45, 50, and 55°C, and pressures ranging from 1 to 500 bar, simulating a wide range of reservoir conditions. Two brine samples representative of different reservoirs were tested, along with mixtures of methane and hydrogen (10 mol% H₂ and 50 mol% H₂) and pure hydrogen. The results show that the solubility curves for pure hydrogen are consistent with literature data, while the impact of brine composition and temperature on solubility was minimal. However, the slope of the solubility curve was more pronounced for gas mixtures with lower hydrogen percentages, indicating that the presence of methane enhances the solubility of hydrogen. The study provides valuable data for estimating the volume of hydrogen that can dissolve in formation water during storage, which is crucial for assessing potential storage losses and the availability of hydrogen for microbial processes. The findings are relevant for the safe and efficient storage of hydrogen in underground reservoirs, particularly in depleted gas reservoirs.