4-Amino-5-thiocarbamoylimidazole (II, thio-AICA) was prepared by direct thiation of 4-amino-5-imidazolecarboxamide (I, AICA) with phosphorus pentasulfide and also by addition of 4-amino-5-imidazole carbonitrile (III) with hydrogen sulfide. Similarly, 5-amino-4-thiocarbamoyl-1-β-D-ribofuranosylimidazole (XVII, thio-AICA-riboside) was prepared by addition of 5-amino-1-(2', 3'-O-isopropylidene-β-D-ribofuranosyl) imidazole-4-carbonitrile (XIII) with hydrogen sulfide followed by removal of the isopropylidene group. XVII was converted by treatment with ethyl formate in ethanolic sodium ethoxide to 6-mercapto-9-β-D-ribofuranosylpurine (XVIII, 6-thioinosine) in good yield and with diethyl carbonate under the similar conditions to 6-mercapto-2-hydroxy-9-β-D-ribofuranosyl-purine (XXI, 6-thioxanthosine). Methylation of the latter with methyl iodide and subsequent amination afforded 6-amino-2-hydroxy-9-β-D-ribofuranosylpurine (XXIII, isoguanosine). In a similar fashion, N⁶-substituted isoguanosines were prepared by reaction of 6-methylthio-2-hydroxy-9-β-D-ribofuranosylpurine (XXII) with the appropriate amines.