The role of various carboxylic acids (substituted/unsubstituted acetic, benzoic and salicylic acids) with various electron donating and/or withdrawing substituents on the Na2WO4/H2WO4-catalysed, biphasic epoxidations of cyclooctene and 1-octene, using H2O2 as terminal oxidant has been investigated. The presence of the different acids in catalytic amounts widely affects the catalytic activity of the above system depending on the nature of their substituents. The results obtained have been discussed in the light of the stereo/electronic effects of these substituents. It has been observed that the added substituted/unsubstituted acetic and benzoic acids generally enhance the rate of the epoxidation while salicylic acids significantly decrease the catalyst activity, under the same conditions, most likely due to the additional deactivating hydroxy moiety. (c) 2006 Elsevier B.V. All rights reserved.