A simple high performance liquid chromatography (HPLC) method was developed and validated for the determination of six phenolic compounds, five anthraquinones (rhein, aloe-emodin, emodin, chrysophanol and physcion) and a flavonoid (kaempferol), in root extracts from Cassia alata L. Solid-phase extraction, using C 18 cartridges, was used to remove interfering substances from the root extracts. The extracts were analyzed on a C 18 column using an isocratic mobile phase which consisted of acetonitrile, methanol, and 10 mM aqueous ammonium acetate (25: 55: 20, v/v). Identification of the analytes was performed by use of standards and on-line mass spectrometric detection using atmospheric pressure chemical ionization. The concentration of the phenolic compounds in the root extracts was determined using HPLC with ultraviolet detection at 260 nm. The limits of detection obtained for the anlytes were in the range of 0.23 -4.61 ppm. The overall R.S.D. precision values (intra and inter-day) for the retention times and peak-areas were lower than 0.16% and 2.10%, respectively. In addition, the recovery of the developed method for the analysis of these phenolic compounds was determined, and ranged from 81.2 ± 4.3% to 106 ± 2%.