Using the dihydroxylation of alkenes as a benchmark, the reactivities of fresh and aged solutions of (NH4)2S2O8 (electrochemically generated) were compared to commercially-procured peroxydisulfate and Oxone®. The study revealed that peroxymonosulfate (Caro’s acid, PMS) is the active oxidant in such reactions. Using complementary redox colorimetry and in situ IR spectroscopy to monitor the oxidants, the competitive decomposition of peroxydisulfate in an acidic solution into PMS and H2O2 can be quantified for the first time. The new insight enabled the design and implementation of both batch and flow processes to maximise the concentration of active PMS oxidant. The utility of these oxidants for organic synthesis is demonstrated by the dihydroxylation of eight styrenes and seven alkyl alkenes, where the ammonium PMS solutions performed better than Oxone® (counterion effect). Last but not least, a non-chromatographic method for isolating and purifying the water-soluble diol product was developed.