Hydrogen-bonded chains in 5-methyl-2-trifluoromethyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one and hydrogen-bonded chains of rings in 5-amino-3-trifluoromethyl-1H-1,2,4-triazole-5-methyl-2-trifluoromethyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one (1/1), the co-crystal of a reaction product and one of its precursors.