A large amount of flavonoid has been extracted and isolated from the heartwood of Prosopis juliflora, an exogenous wood species of Kenya. Structural and physicochemical elucidation based on FTIR, (1)H and (13)C NMR, GC-MS and HPLC analysis clearly demonstrated the presence of (-)-mesquitol as the sole compound without any noticeable impurities. The product was able to slow down oxidation of methyl linoleate induced by AIBN. The important amount and high purity of (-)-mesquitol present in the acetonic extract of P. juliflora could therefore be of valuable interest as a potential source of antioxidants from a renewable origin.