In the ongoing energy transition era, depleted gas reservoirs (DGR) are considered valuable candidates for underground hydrogen and carbon dioxide storage, offering large storage capacities and geological containment. However, the successful implementation of these technologies relies on the ability to accurately predict fluids’ dynamics behaviour at reservoir conditions. This work aims to assess the impact of different Equations of State (EoSs) on the results when simulating underground H2 and CO2 storage. Furthermore, a Material Balance Analysis (MBA) using the p/z method is performed in order to evaluate the accuracy of gas storage predictions by comparing the results obtained from the simulator with those derived from MBA.