Six flavone glycosides were separated by HPLC using RP-18 with a quaternary mobile phase mixture [water-acetonitrile-tetrahydrofuran-acetic acid (80:16:3:1, v/v)]. This procedure was applied to the separation and determination of the most important flavanone glycosides contained in pure grapefruit and sour orange juices and mixtures. This method can be used for quality control of industrial concentrates and juices.