Five new peaks of potential impurities, ranging from 0.05 to 0.12% on an area-percent basis, have been observed in reversed-phase high-performance liquid chromatography of capecitabine drug substance. On the basis of comparison of relative retention times and mass values two of the impurities were identified as intermediates in the process used for synthesis of capecitabine and were characterized as (2-(4-amino-5-fluoro-2-oxopyrimidin-1(2 H )-yl)-5-methyltetrahydrofuran-3,4-diyl diacetate and 2-(5-fluoro-2-oxo-4-(pentyloxycarbonylamino)pyrimidin-1(2 H )-yl)-5-methyl tetrahydrofuran-3,4-diyl diacetate. The area percentages of two of the other peaks increased substantially when the drug was treated with acid and base. The three compounds other than the two intermediates were isolated by preparative HPLC with gradient elution and were characterized by use of spectroscopic techniques, viz. MS-MS, FTIR, and NMR ( 1 H, 13 C and DEPT). On the basis of the spectral data, the structures 5?-deoxy-5-fluorocytidine, 5?-deoxy-5-fluorouridine, and 2-(4-(bis(pentyloxycarbonyl)amino)-5-fluoro-2-oxopyrimidin-(2 H )-yl)-5-methyltetrahydrofuran-3,4-diyl diacetate were proposed for the three impurities.