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Abstract— The kinetic rate equation for the photoreversible photoisomerization between trans -urocanic acid (UA) and cis-UA following absorption of UV radiation has been solved and used to show that the maximum of the trans -UA photoisomerization action spectrum depends upon both the initial trans-UA concentration irradiated in an experiment and upon the irradiation dose used to determine the amount of cis-UA generated. This kinetic model can be used to explain current discrepancies between in vitro and in vivo photoisomerization action spectra reported in the literature.