Ketoprofen is known to induce photosensitivity due to its specific structure and electronic features, and this limits its use in medical applications, In this Article, the photochemistry of (S)-ketoprofen has been investigated by time-resolved resonance Raman spectroscopy to gain additional information so as to better elucidate the possible photochemical reaction mechanism of ketoprofen in different solvents, In nonaqueous solvents like neat acetonitrile and isopropyl alcohol, and 1:1 acetonitrile:water and 1:1 acetonitrile:acidic water aqueous solvents, (S)-ketoprofen exhibits benzophenone-like photochemistry to produce a triplet state, which in turn produces a ketyl radical-like species that then undergoes a cross-coupling reaction with either a dimethyl radical (which is generated by hydrogen abstraction of isopropyl alcohol) or a water molecule, respectively, at the para-position to form a transient species that has a lifetime up to the microsecond time scale. However, photolysis of (,S)-ketoprofen in a 1:1 acetonitrile:alkaline water solution and 3:7 acetonitrile:phosphate buffered solution appears to undergo a prompt decarboxylation reaction. Only one species was observed in the nanosecond time-resolved resonance Raman experiments under these conditions, and this species was tentatively assigned to be a triplet protonated biradical carbanion.