Abstract In this study, we show that the derivatization of some residues encountered (amines, oximes, thiols and phenols) using 3 isopropenyl-α,α-dimethylbenzylisocanate is possible by using aqueous–organic solutions instead of anhydrous solutions. The water content of the reaction solution significantly increases the derivatization yield of thiols and phenols, without considerably modifying that of amines and oximes. These effects can be explained by the cohesive energy of water, which compresses the hydrophobic aggregates formed in the aqueous/organic solvent. These aggregates cannot be dissociated from that of the triethylamine used as a catalyst. Therefore, derivatization in a water/acetonitrile mixture (1:1) in the presence of triethylamine proved to be suitable for measuring trace amounts of many pesticides. The yields obtained were constant between 10 −3 and 10 −7 M nucleophile concentration with excess amounts of derivatizing agents ([isocyanate]/[nucleophilic compound]) ranging from 2.5 to 25 000.