This data set is an analysis of methyl chloride (CH3Cl) and methyl bromide (CH3Br) in Antarctic ice core samples. Investigators reported mixing ratios of methyl chloride gas extracted from samples taken from the South Pole Remote Earth Science and Seismological Observatory (SPRESSO) core, drilled as part of the International Trans Antarctic Science Expedition (ITASE). This data covers an age range of 2159 - 140 years before present (Y.B.P.) where the year 2000 was used as present. Investigators analyzed trace gases in ice core samples from Siple Dome, West Antarctica (dry-drilled C core and deep, fluid-drilled A core) and from South Pole, Antarctica (300 m dry drilled SPRESSO core). Data are available in Microsoft Excel format and are available via FTP.