3-Methyl - 4 - amino - 5 - mercapto - 1,2,4 - triazole forms a red water-soluble complex with Pt(IV) in acidic buffer. The complex exhibits absorption at 400 nm with a molar absorptivity of 54500 1 mol(-1) cm(-1), and Beer＇s law holds over the concentration range of 1.2-17.8 ppm.