An aqueous thiyl radical-based molecular catalytic system was studied for cis-trans isomerization of maleic acid to fumaric acid at benign reaction conditions with no use of organic solvents. At reaction temperatures higher than 317 K, the initial addition of thiol/thiyl to maleate is the rate-determining step, whereas the kinetic bottleneck switches to the subsequent elimination with fumarate formation at lower temperatures. Complementary kinetic analysis and DFT calculations support the catalytic reaction proceeds via the radical mechanism, with an activation energy of 50-70 kJ mol -1 for the initial thiyl radical insertion. Chemical mutational investigations showed the structural factors required for better catalysis of the reaction. While thiols are abundant, their application in artificial catalysis has been limited. This work establishes a solid and fundamental basis for better controlling benign thiyl radical chemistry, offering a promising pathway for advancing organic radical-based chemical conversions.