Mixed-substrate conversions are an under-regarded option to fix carbon dioxide in significant amounts. In such a conversion, carbon dioxide together with one other carbon source such as glucose is converted to a single carbon product. With mixed-substrate conversions, it is possible to incorporate carbon dioxide into products with higher oxidation states than the co-substrate. Using abundant co-substrates such as glucose, glycerol or methanol, it is possible to produce organic acids anaerobically, using CO2 both as an electron acceptor and as an additional carbon source. Here, we outline the thermodynamic feasibility to produce industrially important organic acids with this approach to provide guidance for future metabolic engineering endeavours.