Synthesis of glycosylthiouracil from glycosylthiourea was carried out by the method for synthesis of glycosylthymine described in Part II of this series. 1-Methyl-3-(3-ethoxyacryloyl) thiourea, obtained from N-methylthiourea and ethyl 3, 3-diethoxyacryloyl chloride, was submitted to pyrimidine cyclization and 1-methyl-2-thiouracil was found to be produced. In accordance with this model experiment, 1-(β-D-glucopyranosyl)-2-thiouracil was obtained from 1-(tetra-O-acetyl-β-D-glucopyranosyl) thiourea and ethyl 3, 3-diethoxyacryloyl chloride, and 1-(β-D-ribofuranosyl)-2-thiouracil (2-thiouridine) from 1-(tri-O-benzoyl-β-D-ribofuranosyl) thiourea.