Tissue-equivalent gases (TEGs), often made of a hydrocarbon, nitrogen, and carbon dioxide, have been employed in microdosimetry for decades. However, data on the first Townsend ionization coefficient (??) in such mixtures are scarce, regardless of the chosen hydrocarbon. In this context, measurements of ?? in a methanebased tissue-equivalent gas (CH4 ??? 64.4%, CO2 ??? 32.4%, and N2 ??? 3.2%) were performed in a uniform field configuration for density-normalized electric fields (E/N) up to 290 Td. The setup adopted in our previous works was improved for operating at low pressures. The modifications introduced in the apparatus and the experimental technique were validated by comparing our results of the first Townsend ionization coefficient in nitrogen, carbon dioxide, and methane with those from the literature and Magboltz simulations. The behavior of ?? in the methane-based TEG was consistent with that observed for pure methane. All the experimental results are included in tabular form in the Supplementary material.Funda????o de Amparo ?? Pesquisa do Estado de S??o Paulo (FAPESP)FAPESP: 02/04697-1, 07/50591-4, 09/51809-9