To produce hyperpolarized fumarate, para-enriched hydrogen gas was rapidly bubbled through a precursor solution in a heated steel reactor. The sample was then ejected into a magnetically shielded chamber, and a magnetic field cycle was used to transfer the 1 H singlet order in fumarate (originating from the parahydrogen protons) into C magnetisation of the carboxylate carbon. In experiments involving a purification step, part of this solution was mixed with a concentrated sodium fumarate solution (to raise the overall fumarate concentration), and then mixed with HCl which lowered the pH and caused fumaric acid to precipitate out of solution. The residual reaction solution was filtered off and the fumaric acid crystals were redissolved in aqueous solution. The precipitation and redissolution steps were carried out in a 100 mT Halbach permanent magnet array to preserve the hyperpolarized spin order throughout. Unless otherwise stated, experiments were performed without isotopic enrichment of the precursor molecule, and the ~2.2% of molecules containing a C spin in the carboxylate position were observed in the NMR experiments. A schematic showing the chemical reaction and experimental apparatus is shown in Fig. . Further experimental details are provided in the Materials and Methods.