Inhibition studies of 4-hydroxyphenylpyruvate dioxygenase (HPPD) with various synthesized 2-o-substituted-benzoyl-and 2-alkanoyl-cyclohexane-1, 3-diones suggest that the presence of a strongly electronegative group at the ortho position and the conformation of the benzene ring moiety on the benzoylcyclohexane-1, 3-dione inhibitors are crucial for potent HPPD inhibition.