This thesis explored the potential of synthetic microbial co-cultures as biocatalysts for the conversion of syngas (CO, H2, and CO2) into medium-chain carboxylic acids. A multidisciplinary approach was taken by combining experimental work and genome-scale metabolic modelling. A co-culture of Anaerotignum neopropionicum and Clostridium kluyveri was shown to produce medium-chain carboxylates (up to C7) from ethanol and CO2 —products of syngas fermentation. By incorporating the acetogen Acetobacterium wieringae strain JM, the novel tri-culture successfully produced C4-C6 carboxylates and higher alcohols from CO alone. Constructing the first genome-scale metabolic models (GEMs) of A. neopropionicum and A. wieringae JM provided novel insights into the metabolism of these species. This study also explored the diverse metabolic capabilities of acetogens in co-cultivation with C. kluyveri, and delved into the metabolism of odd-chain elongation by the latter. In conclusion, this thesis highlighted the potential of synthetic co-cultures to expand the product spectrum of syngas fermentation.