High-performance liquid chromatography was used to analyse 1-carboxy-5,7-dibromo-6-hydroxy-2,3,4-trichloroxanthone (HXCA) in 34 commercial batches of U.S. certified color additives D&C Red Nos. 27 and 28 (phloxine B). Thirty-one samples were found to contain HXCA in amounts ranging from 0.01 to 0.47% with an average value of 0.12%. These values are one order of magnitude lower than those previously reported for the Japanese counterpart of D&C Red No. 28 (Food Red No. 104). The analyses also revealed significant differences in the level of HXCA across batches from the same manufacturer. The analyses suggest that the presence of HXCA in D&C Red Nos. 27 and 28 may be avoided in the manufacturing process. A theoretical account that could explain the origin of HXCA in these color additives is also proposed.