Photochemical reactions of metamitron (a 4-amino substituted as-triazin-5(4H)-one) were investigated including UV- and phosphorescence spectroscopy, determination of quantum yields, comparison with measurements in sunlight and products found. The photolysis of metamitron is found to be strongly dependent on solvent and oxygen, as known for other 4-amino-triazin-5(4H)-ones. No photoreaction was found in methanol, acetonitrile and hexane. Photolysis rate constants were obtained using a xenon light source by analysis of the UV-spectra and HPLC. Quantum yields found in water and at pH = 4 and 9 are comparable (Φ = 0.014-0.018) and at pH = 7 about a factor of 2 higher, probably influenced by the phosphate buffer used. Photochemical mechanisms are discussed. With the strong indication of a highly probably pH-independent quantum yield found, the involvement of hydroperoxyl radicals in the first step of the photoreaction, discussed before, is assumed to be unlikely. Besides desamino metamitron a second, minor (and by GC/MS and UV-spectra tentatively assigned) ring cleavage product was found.