The dependence of the quantum yield of metamitron on oxygen concentration in the range c_O2  = (0.005–12.6)· 10⁻⁴ M in aqueous solution was investigated with a polychromatic Xe-light source in the wavelength region 270–376 nm. The quantum yield of Φ = 0.022 ± 0.002 mol mol⁻¹ at T = (293 ± 3) K and c_O2 =2.8·10⁻⁴ M is independent from metamitron concentration in the range of c_M = 8 ·10⁻⁸ − 1.8 · 10⁻⁴ M. A weak temperature dependence of the photolysis of metamitron at constant oxygen concentration was found with an activation energy of E_A = (3 ± 1) kJ mol⁻¹. The yield (ν = 0.91) of the main product desamino-metamitron does not depend on start concentration of metamitron, oxygen concentration or temperature (in the range of T = 281.5–327 K). Oxygen is necessary in the photolysis of metamitron. Quantum yields for the photoreaction of metamitron increase from practically zero at low oxygen concentrations (Φ = 0.002 mol mol⁻¹ at c_O2 −6 M) to a maximum of Φ_max = 0.042 mol mol⁻¹ at c_O2 =5.1·10⁻⁵ M and declines with increasing oxygen concentration to a value of Φ = 0.006 mol mol⁻¹ in oxygen saturated solutions. Two reaction sequences with a reaction from the triplet state T₁ and from a recently proposed transient triplet state T₁′ were used to explain the oxygen-dependent rate constants of metamitron. Both sequences explain the oxygen dependence equally well and it was not possible to distinguish between both mechanisms. Aqueous solutions of metamitron are recommended as a promising actinometric system, especially suitable for measurements in sunlight.