From the formation of 3-methyl-2-thiothymine or 3-methyl-2-thiouracil from 1-methylthiourea and ethyl 2-formylpropionate or ethyl 3, 3-diethoxypropionate, the same reaction was applied to 1-(poly-O-acyl-β-D-glycosyl) thiourea and -urea, and-3-(β-D-glucopyranosyl) uracil, 3-(β-D-glucopyranosyl)-2-thiouracil 3-(β-D-glucopyranosyl)-2-thiothymine, and 3-(β-D-ribofuranosyl)-2-thiouracil (2-isothiouridine) were obtained. These compounds have the glycosyl group bonded to the nitrogen in 3-position of the pyrimidine ring, as against that in the nitrogen in 1-position in pyrimidine nucleoside, and was tentatively designated as isopyrimidine nucleoside. The structure of these isopyrimidine nucleosides was established and it was revealed that these iso compounds have specific nature of being extremely labile to acids.