We estimate the quantum yields of photochemical conversion of differently structured psoralens in water and alcohol. In water, the psoralen and angelicin during illumination at 313 nm wavelength are found to photodissociate with quantum yields of 0.01 ± 0.002 and 0.025 ± 0.005, respectively. In alcohol, the photochemical stability of psoralen and angelicin increases abruptly. Even in water solution, 5-methoxy psoralen and 8-methoxy psoralen are stable to ultraviolet illumination. We studied the psoralen properties in water solutions and suggested main pathways of psoralen photochemical destruction.