Trichloroacetic anhydride is reported to be an efficient derivatization reagent, since it produces stable derivatives having both a higher mass fragmentation pattern and lesser volatility than derivatives of trifluoroacetic anhydride (TFAA) or heptafluorobutyric anhydride (HFBA). The interaction of thymidine 5′-phosphate with trichloroacetic anhydride in dimethylformamide and acetonitrile in the presence of tertiary amines hs been studied.