Characterizing the wettability of hydrogen (H 2)–methane (CH 4) mixtures in subsurface reservoirs is the first step towards understanding containment and transport properties for underground hydrogen storage (UHS). In this study, we investigate the static contact angles of H 2 –CH 4 mixtures, in contact with brine and Bentheimer sandstone rock using a captive-bubble cell device at different pressures, temperatures and brine salinity values. It is found that, under the studied conditions, H 2 and CH 4 show comparable wettability behaviour with contact angles ranging between [25°–45°]; and consequently their mixtures behave similar to the pure gas systems, independent of composition, pressure, temperature and salinity. For the system at rest, the acting buoyancy and surface forces allow for theoretical sensitivity analysis for the captive-bubble cell approach to characterize the wettability. Moreover, it is theoretically validated that under similar Bond numbers and similar bubble sizes, the contact angles of H 2 and CH 4 bubbles and their mixtures are indeed comparable. Consequently, in large-scale subsurface storage systems where buoyancy and capillary are the main acting forces, H 2 , CH 4 and their mixtures will have similar wettability characteristics. [Display omitted] • Static contact angles for H 2 –CH 4 /brine/rock systems were directly measured. • All gas mixtures showed comparable behaviour and no P, T or salinity dependency was found. • A theoretical validation of the results is presented based on the Young Laplace Eq. • For similar N B O and bubble sizes, the contact angles for H 2 and CH 4 are comparable. [ABSTRACT FROM AUTHOR]