Off-gas may refer to: Outgassing air pollution exhaust gas flue gas the effluent gas of a chemical reactor "off-gas" can also refer to byproduct gases from chemical processing such as Thiol (mercaptan) oxidation "Refinery Off-Gas" ("ROG") is a gas mixture with varying composition (usually hydrogen-rich), a common byproduct of Catalytic reforming and Hydrodesulfurization (hydrotreating) units