NOVELTY - A bioreactor system for fermentation of a gaseous substrate includes (a) at least one fermentation vessel, configured such that in use, liquid fermentation broth can be circulated in a loop; (b) at least one pumping means configured to circulate fermentation broth and optionally the gaseous substrate gas around the loop; (c) at least one gas inlet configured to direct gaseous substrate into the vessel; and (d) at least one gas outlet configured to allow gas to exit the vessel.    USE - The bioreactor system and gas/liquid contact module are used for fermentation of gaseous substrates to produce one or more products (claimed).    ADVANTAGE - The bioreactor system and gas/liquid contact module are used for improved and efficient microbial fermentation of gaseous substrates.    DETAILED DESCRIPTION - A bioreactor system for fermentation of a gaseous substrate comprising carbon monoxide (CO) and optionally hydrogen gas (H2), or carbon dioxide (CO2) and H2, to one or more products, the system includes (a) at least one fermentation vessel, configured such that in use, liquid fermentation broth can be circulated in a loop; (b) at least one pumping means, configured to, in use, circulate fermentation broth and optionally the gaseous substrate gas around the loop; (c) at least one gas inlet configured to, in use, direct gaseous substrate into the vessel; and (d) at least one gas outlet, configured to, in use, allow gas to exit the vessel. INDEPENDENT CLAIMS are:    (1) a gas/liquid contact module configured for fermentation of gaseous substrates to produce one or more products in a liquid fermentation broth, where the contact module comprises multiple channels; and    (2) a method of fermentation of gaseous substrates to produce products in a liquid fermentation broth by providing a gaseous substrate to a circulated liquid loop bioreactor containing a culture of one or more microorganisms; and fermenting the culture in the bioreactor to produce one or more products from the substrate.