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Molecular hydrogen (H₂), its stable isotope signature (δD), and the key combustion parameters carbon monoxide (CO), carbon dioxide (CO₂), and methane (CH₄) were measured from various combustion processes. H₂ in the exhaust of gas and oil-fired heaters and of waste incinerator plants was generally depleted compared to ambient intake air, while CO was significantly elevated. These findings contradict the often assumed co-occurring net H₂ and CO emissions in combustion processes and suggest that previous H₂ emissions from combustion may have been overestimated when scaled to CO emissions. For the gas and oil-fired heater exhausts, H₂ and δD generally decrease with increasing CO₂, from ambient values of ~0.5 ppm and +130‰ to 0.2 ppm and −206‰, respectively. These results are interpreted as a combination of an isotopically light H₂ source from fossil fuel combustion and a D/H kinetic isotope fractionation of hydrogen in the advected ambient air during its partial removal during combustion. Diesel exhaust measurements from dynamometer test stand driving cycles show elevated H₂ and CO emissions during cold-start and some acceleration phases. While H₂ and CO emissions from diesel vehicles are known to be significantly less than those from gasoline vehicles (on a fuel-energy base), we find that their molar H₂/CO ratios (median 0.026, interpercentile range 0.12) are also significantly less compared to gasoline vehicle exhaust. Using H₂/CO emission ratios, along with CO global emission inventories, we estimate global H₂ emissions for 2000, 2005, and 2010. For road transportation (gasoline and diesel), we calculate 8.3 ± 2.2 Tg, 6.0 ± 1.5 Tg, and 3.8 ± 0.94 Tg, respectively, whereas the contribution from diesel vehicles is low (0.9–1.4%). Other fossil fuel emissions are believed to be negligible but H₂ emissions from coal combustion are unknown. For residential (domestic) emissions, which are likely dominated by biofuel combustion, emissions for the same years are estimated at 2.7 ± 0.7 Tg, 2.8 ± 0.7 Tg, and 3.0 ± 0.8 Tg, respectively. For biomass burning H₂ emissions, we derive a mole fraction ratio ΔH₂/ΔCH₄ (background mole fractions subtracted) of 3.6 using wildfire emission data from the literature and support these findings with our wood combustion results. When combining this ratio with CH₄ emission inventories, the resulting global biomass burning H₂ emissions agree well with published global H₂ emissions, suggesting that CH₄ emissions may be a good proxy for biomass burning H₂ emissions.