Molecular hydrogen (H<sub>2</sub>), its stable isotope signature (δD), and the key combustion parameters carbon monoxide (CO), carbon dioxide (CO<sub>2</sub>), and methane (CH<sub>4</sub>) were measured from various combustion processes. H<sub>2</sub> 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 H<sub>2</sub> and CO emissions in combustion processes and suggest that previous H<sub>2</sub> emissions from combustion may have been overestimated when scaled to CO emissions. For the gas and oil-fired heater exhausts, H<sub>2</sub> and δD generally decrease with increasing CO<sub>2</sub>, 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 H<sub>2</sub> 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 H<sub>2</sub> and CO emissions during cold-start and some acceleration phases. While H<sub>2</sub> 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 H<sub>2</sub>/CO ratios (median 0.026, interpercentile range 0.12) are also significantly less compared to gasoline vehicle exhaust. Using H<sub>2</sub>/CO emission ratios, along with CO global emission inventories, we estimate global H<sub>2</sub> 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 H<sub>2</sub> 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 H<sub>2</sub> emissions, we derive a mole fraction ratio ΔH<sub>2</sub>/ΔCH<sub>4</sub> (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 CH<sub>4</sub> emission inventories, the resulting global biomass burning H<sub>2</sub> emissions agree well with published global H<sub>2</sub> emissions, suggesting that CH<sub>4</sub> emissions may be a good proxy for biomass burning H<sub>2</sub> emissions.