There are significant questions regarding the sources and sinks of atmospheric trace gases including methane and hydrogen. These uncertainties complicate efforts to predict climate change. Isotopic analysis provides additional information about trace gas sources, sinks and reactions that is not available from other methods. Using a combination of laboratory kinetics, spectroscopy and theory we have mapped the deuterium isotope effects in the atmospheric oxidation reactions of methane and can now provide the first detailed picture of deuterium propagation in the atmosphere. Points of interest include the branching ratio for hydrogen abstraction by O₂ from CH₂DO and the pressure dependence of the kinetic isotope effect when formaldehyde is photolyzed to produce molecular hydrogen. Deuterium isotope effects are large, giving an easily measured parameter. We describe how deuterium enrichments in the field will be sensitive to pressure, temperature, actinic flux and the oxidant field.