An HPLC method for determination of very low concentrations of 4-aminophenol was elaborated, and the kinetics of its decomposition at low temperatures and at laboratory temperature in the absence of light or under irradiation at 254 nm were compared. To increase the sensitivity of the method, derivatization of 4-aminophenol with various reagents was tested. Phthalaldehyde proved to be the best; its reaction at laboratory temperature in the presence of 2-sulfanylethanol is very fast. The derivatization conditions were optimized.