The photodegradation of the herbicide clomazone in the presence of S2O82− or of humic substances of different origin was investigated. A value of (9.4 ± 0.4) × 108 m−1 s−1 was measured for the bimolecular rate constant for the reaction of sulfate radicals with clomazone in flash-photolysis experiments. Steady state photolysis of peroxydisulfate, leading to the formation of the sulfate radicals, in the presence of clomazone was shown to be an efficient photodegradation method of the herbicide. This is a relevant result regarding the in situ chemical oxidation procedures involving peroxydisulfate as the oxidant. The main reaction products are 2-chlorobenzylalcohol and 2-chlorobenzaldehyde. The degradation kinetics of clomazone was also studied under steady state conditions induced by photolysis of Aldrich humic acid or a vermicompost extract (VCE). The results indicate that singlet oxygen is the main species responsible for clomazone degradation. The quantum yield of O2(a1Δg) generation (λ = 400 nm) for the VCE in D2O, ΦΔ = (1.3 ± 0.1) × 10−3, was determined by measuring the O2(a1Δg) phosphorescence at 1270 nm. The value of the overall quenching constant of O2(a1Δg) by clomazone was found to be (5.7 ± 0.3) × 107 m−1 s−1 in D2O. The bimolecular rate constant for the reaction of clomazone with singlet oxygen was kr = (5.4 ± 0.1) × 107 m−1 s−1, which means that the quenching process is mainly reactive.