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Jurisdictions are looking into mixing hydrogen into the natural gas (NG) system to reduce greenhouse gas (GHG) emissions. Earlier studies have focused on well-to-wheel analysis of H 2 fuel cell vehicles, using high-level estimates for transportation-based emissions. There is limited research on transportation emissions of hythane, a blend of H 2 and NG used for combustion. An in-depth analysis of the pipeline transportation system was performed for hythane and includes sensitivity and uncertainty analyses. When hythane with 15% H 2 is used, transportation GHG emissions (gCO2eq/GJ) increase by 8%, combustion GHG emissions (gCO2eq/GJ) decrease by 5%, and pipeline energy capacity (GJ/hr) decreases by 11% for 50–100 million m3/d pipelines. Well-to-combustion (WTC) emissions increase by 2.0% without CCS, stay the same with a 41% CCS rate, decrease by 2.8% for the 100% CCS scenario, and decrease by 3.6% in the optimal CO 2 -free scenario. While hythane contains 15% H 2 by volume only 5% of the gas' energy comes from H 2 , limiting its GHG benefit. [Display omitted] • A model is developed to assess well-to-combustion (WTC) greenhouse gas (GHG) emissions of hythane. • A hythane blend of 15% H 2 and 85% natural gas is assessed. • Transportation GHG emissions increase by 8% and combustion GHG emissions decrease by 4%. • Without carbon capture, WTC GHG emissions increase by 2%. • Steam methane reforming of hydrogen with 90% carbon capture will reduce WTC GHG emissions by 3%. [ABSTRACT FROM AUTHOR]