The use of a C6 to C24 mono- and polyunsaturated acids or their derivatives thereof as enzyme inhibitors particularly as P450 and esterase inhibitors in pesticide formulations. Also claimed is a pesticide formulation comprising C6 to C24 mono- and polyunsaturated acids or their derivatives thereof and a pesticide. The acids may obtained from naturally occurring product, such as vegetable oils, soya, acorns, chestnuts, horse chestnuts and coffee. The exemplified compounds are potassium salts of fatty acids (C14-C18) obtained as a by-product of olive oil production. The pesticide may a synthetic pyrethroid, the exemplified pesticide is alpha-cypermethrin.
