Three commercial pendimethalin capsule suspension (CS) formulations were obtained from the Egyptian market, produced by three distinct companies to examine the impact of storage at room temperature and sunlight exposure for six months, as well as storage at 54 ±2 °C for 70 days, on the stability of pendimethalin. Furthermore, GC-MS was employed to identify some breakdown products following exposure to sunlight. According to the obtained results, pendimethalin was stable after storage at room temperature and 54 ±2 °C for 14 days, and the degradation rate was not affected even with increasing the storage period for all sources. Pendimethalin was less stable after being exposed to sunlight than storage at 54 ±2 °C, and there was no difference in the degradation rate for all pendimethalin formulations. Pendimethalin photodecomposes by oxidative dealkylation and nitro reduction. Four degradation products were found using GC-MS analysis of samples exposed to the sunlight as follows: N-(1-ethylpropyl)-3-methyl-2,6-dinitroaniline, N-propyl-3,4-dimethyl-2,6-dinitroaniline, 4,5-dimethyl-3-nitro-N2-(pentan-3-yl)benzene-1,2-diamine and 2,6-dinitro-3,4-dimethyalaniline.