This data set of half-hourly atmospheric methane (CH4) mole fractions and 222Radon activity concentrations continuously measured in Heidelberg, Germany, from 1996-1998 at INF 366 and from 2000-2020 at INF 229 forms the basis of CH4 flux estimates in the Heidelberg footprint using the Radon Tracer Method (Levin et al., https://doi.org/10.5194/acp-21-17907-2021, 2021). The CH4 data from 1996-2017 were measured by gas chromatography while the last three years of measurements (2018-2020) were conducted by Cavity RingDown Spectroscopy. All data have been quality checked for instrumental failures. They are reported on the WMO X2004A mole fraction scale. All 222Radon activity concentrations were measured with the static filter system as described by Levin et al. (https://DOI: 10.3402/tellusb.v54i5.16681, 2002). They have been corrected for disequilibrium between reported 222Radon and measured 214Polonium activity concentration and brought to a preliminary ANSTO scale by multiplication with an overall factor of 1.23 according to Schmithüsen et al. (https://doi:10.5194/amt-10-1299-2017, 2017). Please cite these data together with the accompanying paper by Levin et al. (https://doi.org/10.5194/acp-21-17907-2021, 2021).