An improved method was developed for the preparation of thiamine[^<32>P]diphosphate ([^<32>P]TDP) and thiamine[^<32>P]monophosphate ([^<32>P]TMP) of the specific radioactivity of 10 and 5 mCi/mmol, respectively. The purity of those preparations was checked by paper chromatography, high voltage paper electrophoresis, thin layer chromatography, autoradiography and gel filtration chromatography, and found to be more than 93% It took 6 days from the start of phosphorylation to the end of purification. The oxidation product of [^<32>P]TDP and [^<32>P]TMP by BrCN, namely [^<32>P]thiochrome diphosphate and monophosphate, were found to be unstable radiologically. Hence they gave a non-fluorescent radioactive peak amounting to more than 15% of total activity when the oxidation product of pure [^<32>P]TDP or[^<32>P]TMP was developed through a gel filtration column without the addition of a radical scavenger.