Sub-quality natural gas (SQNG) is defined as natural gas whose composition exceeds pipeline specifications of nitrogen, carbon dioxide (CO2) and/or hydrogen sulfide (H2S). Approximately one-third of the U.S. natural gas resource is sub-quality gas [1]. Due to the high cost of removing H2S from hydrocarbons using current processing technologies, SQNG wells are often capped and the gas remains in the ground. We propose and analyze a two-step hydrogen production scheme using SQNG as feedstock. The first step of the process involves hydrocarbon processing (via steam-methane reformation, autothermal steam-methane reformation, pyrolysis and autothermal pyrolysis) in the presence of H2S. Our analyses reveal that H2S existing in SQNG is stable and can be considered as an inert gas. No sulfur dioxide (SO2) and/or sulfur trioxide (SO3) is formed from the introduction of oxygen to SQNG. In the second step, after the separation of hydrogen from the main stream, un-reacted H2S is used to reform the remaining methane, generating more hydrogen and carbon disulfide (CS2). Thermodynamic analyses on SQNG feedstock containing up to 10% (v/v) H2S have shown that no H2S separation is required in this process. The Part I of this paper includes only thermodynamic analyses for SQNG pyrolysis and autothermal pyrolysis. (c) 2006 Elsevier B.V. All rights reserved.