The photochemistry of difloxacin in acetonitrile was investigated using laser flash photolysis. Energy transfer method was also used to estimate the properties of the triplet state of difloxacin. The results showed that excitation of the solution containing difloxacin and naproxen upon 355 nm laser flash produced the triplet of difloxacin at 580 nm firstly. Subsequently, the excitation energy of triplet difloxacin was transferred to naproxen generating triplet naproxen. Characteristic absorption of the triplet state of naproxen at 440 nm was recorded. By means of transfer of excitation energy, the molar absorption coefficients of triplet difloxacin were determined to be 8,900 dm3/mol?cm at 580 nm. Through changing the concentration of difloxacin, the self-decay rate constant and triplet lifetime for difloxacin in acetonitrile solution were observed to be 5.39 × 106 s−1 and 2.8 μs.Cited