This study investigates the solubility of hydrogen (H₂) and its mixtures with methane (CH₄) in brine under underground hydrogen storage conditions. The research aims to estimate the amount of hydrogen that can dissolve in formation water during storage in depleted gas reservoirs, which is crucial for assessing potential storage losses and quantifying hydrogen available for microbial processes. A PVT cell was used to measure the solubility of hydrogen and its mixtures in saline water at temperatures of 45°C, 50°C, and 55°C, and pressures ranging from 1 bar to 500 bar. Two brine samples, representative of different reservoirs, were tested, along with two hydrogen-methane mixtures (10 mol% and 50 mol% H₂) and pure hydrogen. The solubility of hydrogen in brine was compared with literature data and models, including the model by Torin-Ollarves and Trusler (2021), which accounts for brine salinity and temperature. The results showed that the solubility of hydrogen in brine is influenced by the composition of the gas mixture, with steeper solubility curves observed for mixtures with lower hydrogen content. The study also found that the impact of brine salinity and temperature on solubility was relatively minor, while the gas mixture composition had a more significant effect. The experimental data were used to estimate the quantity of gas that dissolves in formation water during a storage cycle, which is essential for understanding hydrogen availability for microbial processes. The results indicate that the solubility of hydrogen in brine is higher when the gas mixture contains a lower percentage of hydrogen. The study also highlights the importance of accurate modeling and experimental data for predicting hydrogen solubility in brine under various conditions, which is critical for the safe and efficient storage of hydrogen in underground reservoirs.This study investigates the solubility of hydrogen (H₂) and its mixtures with methane (CH₄) in brine under underground hydrogen storage conditions. The research aims to estimate the amount of hydrogen that can dissolve in formation water during storage in depleted gas reservoirs, which is crucial for assessing potential storage losses and quantifying hydrogen available for microbial processes. A PVT cell was used to measure the solubility of hydrogen and its mixtures in saline water at temperatures of 45°C, 50°C, and 55°C, and pressures ranging from 1 bar to 500 bar. Two brine samples, representative of different reservoirs, were tested, along with two hydrogen-methane mixtures (10 mol% and 50 mol% H₂) and pure hydrogen. The solubility of hydrogen in brine was compared with literature data and models, including the model by Torin-Ollarves and Trusler (2021), which accounts for brine salinity and temperature. The results showed that the solubility of hydrogen in brine is influenced by the composition of the gas mixture, with steeper solubility curves observed for mixtures with lower hydrogen content. The study also found that the impact of brine salinity and temperature on solubility was relatively minor, while the gas mixture composition had a more significant effect. The experimental data were used to estimate the quantity of gas that dissolves in formation water during a storage cycle, which is essential for understanding hydrogen availability for microbial processes. The results indicate that the solubility of hydrogen in brine is higher when the gas mixture contains a lower percentage of hydrogen. The study also highlights the importance of accurate modeling and experimental data for predicting hydrogen solubility in brine under various conditions, which is critical for the safe and efficient storage of hydrogen in underground reservoirs.