The deuterium (D) component of the global molecular hydrogen (H2) cycle is discussed. Field measurements of the fractionation effect during soil uptake of atmospheric H2 (αsoil) were made using an in situ static flux chamber, and an αsoil of 0.943¿0.024 (¿1&sgr;) was determined. The ΔD of H2 emitted during combustion of pine needles and pine branches was experimentally measured at -330¿50? (versus Vienna SMOW) and was 230¿60? lower than the ΔD of the plant matter combusted. This hydrogen isotope fractionation effect yielded an estimate of -290¿60? for the ΔD of H2 emitted during global biomass burning. The ΔD of H2 emitted by fossil fuel combustion in the Seattle, Washington, urban area was -196¿10?. On the basis of a global H2 budget calculation, a ΔD of +130¿70? is required for the photochemical H2 source in order for the available isotopic measurements to agree with the global H2 budget. A possible mechanism for a significant enrichment in the deuterium content of H2 produced by the photochemical destruction of methane is given. We suggest that the currently available isotopic measurements are not in disagreement with the current view of the global H2 budget. ¿ 2001 American Geophysical Union