N-bromoacetyl-3,3', 5-triiodo-L-thyronine (BrAcT3) was synthesized in a one step procedure and purified by countercurrent chromatography (CCC). The CCC separation produced an unusually sharp peak of BrAcT3. Further investigation indicated that bromoacetic acid present in the crude reaction mixture was responsible for sharpening the BrAcT3 peak by making a pH gradient in the effluent of CCC.