The photocatalytic degradation of a herbicide derivative, Dinoterb (1), has been investigated in aqueous micellar solution in the presence of titanium dioxide (TiO2) and air as a function of irradiation time under a variety of conditions using UV and HPLC analysis techniques. The degradation kinetics was studied under different conditions such as different types of TiO2, catalyst concentration, substrate concentration and reaction pH in the presence of air. The photocatalyst Degussa P25 was found to be more efficient catalyst as compared to other photocatalysts tested. The model compound was found to degrade more efficiently under neutral pH as compared to acidic and alkaline pH. GC/MS analysis of the irradiated samples indicate the formation of 2-isopropyl-4,6-dinitro-phenol as by-product which has been characterized on the basis of molecular ion and mass fragmentation pattern. A probable pathway for its formation has been proposed.