4-Amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one (metamitron) and 4-amino-6-methyl-3-phenyl-1,2,4-triazin-5(4H)-one (isometamitron).

The title structures, both C(10)H(10)N(4)O, are substitutional isomers. The N-N bond lengths are longer and the C=N bond lengths are shorter by ca 0.025 A than the respective average values in the C=N-N=C group of asymmetric triazines; the assessed respective bond orders are 1.3 and 1.7. There are N-H...O and N-H...N hydrogen bonds in both structures, with 4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one containing a rare bifurcated N-H...N,N hydrogen bond. The structures differ in their molecular stacking and the hydrogen-bonding patterns.