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The thermal reaction rate of muonium with methane (and ethane) in the gas phase Muon polarizations are reported for nitrogen and ethane over a wide pressure range from below 1 to 200 atm for N z and up to 245 atm for C Z H 6 • The N2 measurements were made at ambient temperature, while those for C 2 H 6 were made at temperatures both above and below the critical temperature (305.3 K). This is the first ,uSR study of muonium and diamagnetic muon formation to cover the entire range from a low pressure gas to densities typical of liquids. The data are discussed in terms of hot atom and spur models. In the lowest pressure " range, below 1.5 atm for N2 and about 10 atm for C Z H 6 , the muonium polarization increases with pressure. This is well understood in terms of epithermal charge exchange. In N2 there is a small diamagnetic fraction, which is ascribed to the NzMu + molecular ion. This fraction approaches zero as the pressure is increased to 200 atm, with a corresponding increase in the muonium fraction, consistent with charge neutralization of the molecular ion by electrons from the radio lysis track. In C 2 H n , there is a decrease in the muonium fraction and a concomitant increase of the diamagnetic fraction with density, the changes occurring in two stages. The initial change is explained by stabilization of the vibrationally excited substitution products of hot muonium reactions. The second one is explained by proton transfer from the molecular ion adduct, C2H6Mu + + C 2 H 6 --+C 2 H s Mu + CzHl, trapping the muon in a diamagnetic product. Both N2 and C 2 H 6 have a missing fraction of polarization above 10 atm, most likely due to spin exchange of Mu with paramagnetic species created in the muon track. In N z , the missing fraction is recovered at pressures beyond about 150 atm, which is explained by scavenging of electrons by positive ions. In C 2 H 6 the missing fraction is roughly constant for densities beyond 5 mol I-I (:::::;50 atm), and about twice the maximum found for N z • Both facts are consistent with the existence of ethyl radicals and hydrogen atoms in ~H6' which are longer lived than the spur electrons.