Structural water molecules in proteins are notoriously hard to detect in solution. Hyperpolarized water with UV radicals as polarizing agents provides the sensitivity and resolution to detect residue...

α-Keto acids readily form radicals upon UV irradiation, shaping their use as radical precursors in dissolution dynamic nuclear polarization (dDNP) and governing their role in atmospheric chemistry. Their rich photochemistry, however, is not captured by current electron paramagnetic resonance (EPR) experimental evidence, and assignment to a single ketyl species is a simplification obscuring their broader radical behavior. To probe this, we combined isotope labeling with 9.5 GHz EPR spectroscopy to monitor irradiation-time-resolved buildup and temperature-dependent decay of UV-generated radicals. High-frequency EPR (94 GHz) and DFT-guided simulations aided spectral deconvolution, revealing at least four distinct radical species originating from pyruvic acid. We identified the dominant monomeric ketyl radical, a methyl radical, a carboxylate radical anion, and a dimeric ketyl species. Thus, α-keto acids exhibit multispecies radical chemistry under UV-irradiation, which should be considered when interpreting solid-state ESR or DNP behavior and atmospheric processes.