The ability to predict melatonin stability during food processing or storage is important. Therefore, the degradation of melatonin in both aqueous solutions and fruit juice samples was investigated. The pH values of aqueous solutions were set over a pH range from 1 to 13 and at four different temperatures (60, 70, 80 and 90 degrees C). The highest remaining melatonin (C-R) was observed in the lowest pH solution (pH = 1, C-R > 65%). Melatonin concentrations decreased with rising pH levels from pH 4 to 13 during storage time. The thermal degradation rate constant of melatonin (k) values obtained followed the order: k(90 degrees C) (0.175) >k(80 degrees C) (0.123) >k(70 degrees C) (0.082) >k(60 degrees C) (0.027). Thermal degradation kinetics followed the first-order reaction model with a high range of coefficients of determination (0.9744 < R-2 < 0.995). The temperature also affected on melatonin degradation in fruit juices which the degradation rate was increased with the presence of light and high temperature. Our results can be used as guidelines to develop a processing method that predicts melatonin degradation during thermal processing of food products.