This dataset comprises high-precision atmospheric methane (CH4) mole fraction measurements and isotopic compositions (δ13C-CH4 and δD-CH4), collected from eight laboratories operating at high-latitude stations in both hemispheres between 1988 and 2023. Each record includes timestamped values (DateTimeISO, Year, Month, Day, Hour, Minute, Second), laboratory identifiers, CH4 mole fractions and their associated uncertainties, isotopic ratios and uncertainties for both Northern and Southern Hemisphere sites, as well as metadata such as the number of data points and quality flags. The dataset has been harmonised using a nearest-neighbour approach and corrected for interlaboratory offsets to produce hemisphere-specific time series aligned to the MPI-BGC methane isotope and mole fraction scale. All data sources are listed in the included data provider listing. The dataset supports B. Dasgupta et al. (2025). Harmonisation of methane isotope ratio measurements from different laboratories using atmospheric samples, submitted to Atmospheric Measurement Techniques.