A variety of new fluorinated chemicals have been prepared for the first time and tested as inhibitors of esterases, one of the main enzymes involved in pheromone catabolism, in two economically important pests, the Egyptian armyworm Spodoptera littoralis (SL) and the Mediterranean corn borer Sesamia nonagrioides (SN). Using the respective major component of the pheromone as substrate, the K(m) and V(max) of the antennal esterase of both insects resulted to be 5.66 x 10(-4) M and 8.47 x 10(-6) Mmin(-1) for SL and 1.61 x 10(-7) M and 1.25 x 10(-7) Mmin(-1) for SN, pointing out that SN esterase has a higher affinity for its corresponding substrate than SL. In general, the trifluoromethyl ketones (TFMKs) exhibited higher inhibitory potency than the corresponding difluoromethyl ketones (DFMKs) or difluoroaldehydes (DFAs). The compounds appeared to hydrate differently in aqueous solution, the extent of hydration following the order: alpha,alpha-DFMKs<alpha,alpha-difluoro-beta-thioalkylmethyl ketones<TFMKs<beta-thiotrifluoromethyl ketones<alpha,alpha-DFAs. No clear correlation has been found between the K(hyd) and the inhibitory potency and no specificity has been found when the chemicals were assayed on extracts of both insects.