Recently in this laboratory 1-substituted-2-thio-4-amino-6-methylformamidino1,3,5-triazines(XIIIa-r) were synthesised by isomerisation of 2-substituted imino-4-amino-6-formamidino-1,3,5-thiadiazines (XIIa-r) successfully by refluxing with 10% aqueous ethanolic sodium bicarbonate medium. The structure of all the synthesized compounds was justified on the basis of chemical characteristics, elemental analysis and spectral studies.