Underground hydrogen storage (UHS) in porous media is expected to play a key role in enabling large scale, long duration energy storage for renewable energy systems. The feasibility of UHS in depleted gas reservoirs depends strongly on multiphase flow behavior and gas trapping during injection and withdrawal cycles. This study experimentally investigates the co - flow of hydrogen, methane, and a 50–50% molar hydrogen methane mixture with 1 % w.t NaCl brine in a Berea sandstone core under UHS relevant conditions. Steady state horizontal core flooding experiments at 18 °C and 100 bar, combined with medical X-ray CT imaging, were used to determine relative permeability (kr) and capillary pressure (Pc) during drainage and imbibition cycles. The results showed that hydrogen exhibited slightly higher kr than methane during drainage, with near end point kr values of 0.0399 and 0.0233, respectively, at a gas fractional flow ( 𝑓𝑔 ) 𝑓𝑔 = 0.99, reflecting hydrogen’s lower viscosity and higher mobility. During imbibition, both methane and the mixture showed marginally higher kr than hydrogen, indicating improved producibility. The mixture displayed intermediate behavior: at 𝑓𝑔 = 0.8 the mixture’s drainage (kr=0.0022) and imbibition 2 ( kr=0.0015) kr values, and Pc (Pc = 0.18–0.41 bar) fell between those of hydrogen (kr = 0.0014 , Pc = 0.22–0.40 bar) and methane (kr = 0.0016, Pc = 0.14–0.33 bar). Hydrogen showed greater hysteresis than either methane or the mixture, with linear trapping coefficients (A) of 0.66–0.459 versus 0.62–0.35 for methane, and 0.15 for the mixture across successive drainage/imbibition cycles. The lower mixture hysteresis suggests higher recovery efficiency than hydrogen gas. The experimentally derived kr and Pc data provide valuable input for UHS modeling, reservoir simulation, and injection/withdrawal optimization. This study concludes that mixtures of hydrogen and methane will not exhibit significantly different kr/Pc under relevant UHS conditions, and simulations may treat the mixed systems similarly to the pure systems. However, experimental errors (including the potential for trapped residue gas from previous gas type experiments in the core) make conclusions from these outcomes problematic and future studies may be required to verify the results.