Novel series of 1-substituted-2-thio-(1H)-4-(1-substitutedthiocarbamido -3-yl)-6-substitutedamino 1,2-dihydro-s-triazines[4a(i) to 4f(iii)] have been obtained by the isomerisation of 2-substitutedamino-4-(1 substitutedthioc- arbamido-3-yl)- 6-substitutedimino-1,3,5-thiadiazines[3a(i) to 3f(iii)] in presence of ethanolic sodium bicarbonate solution, which have been obtained by basification of their hydrochlorides [2a(i) to 2f(iii)] which are synthesized by the interaction of 1,3-Bis-(N- substitutedthioamido) guanidine (1a-f) and N-aryl/alkylisocyanodichlorides.The latter were prepared initially by the condensation of guanidine carbonate and N-substitutedisothiocyanate. The structure of all these compounds was established on the basis of elemental analysis, IR and PMR spectral data.