BACKGROUND Clomazone is a popular herbicide used on California rice fields and exhibits rapid anaerobic microbial degradation (t(1/2) = 7.9 days). To test the potential of direct and indirect photolytic degradation as a cofactor in the overall degradation rate, sacrificial time-series microcosms were amended with water, non-sterilized soil + water and sterilized soil + water. Clomazone was added to each microcosm, which was then exposed to natural and artificial sunlight over 35 days. Water and acetonitrile extracts were analyzed for clomazone and metabolites via LC/MS/MS. RESULTS The calculated pseudo-first-order degradation rate constants (k) were k(water) = 0-0.005 ± 0.003 day(-1) , k(sterile) = 0-0.005 ± 0.003 day(-1) and k(non-sterile) = 0.010 ± 0.002-0.044 ± 0.007 day(-1) , depending on light type. The formation of ring-open clomazone, a microbial metabolite, correlated with clomazone degradation. Trace amounts of 5-hydroxyclomazone (m/z = 256 → 125), aromatic hydroxyclomazone (m/z = 256 → 141) and an unknown product (m/z = 268 → 125) were observed. CONCLUSIONS The photolytic degradation rate depends on both light type and the quality of the chromophores that induce indirect photolysis. Microbial degradation was found to be sensitive to temperature fluctuations. Overall, microbes are shown to be more detrimental to the environmental fate of clomazone than photolysis.