Limonene epoxidation by hydrogen peroxide in the presence of Keggin-type heteropolycompound supported catalysts was studied. Catalysts containing molybdenum or tungsten heteropolycompounds were prepared by pore filling impregnation of carbon or alumina, and were characterized by diffuse reflectance spectroscopy (DRS), Fourier transform infrared (FT-IR) and potentiometric titration. During the reaction, limonene oxide, carvone, carveol, diols and limonene hydroperoxide were produced. Different reaction conditions have been studied, such as nitrogen atmosphere, absence of light, and radical scavenger addition. A catalytic process of oxygen transference could be assumed to occur when a lacunar tungstophosphate supported on carbon is used as catalyst, forming a peroxometalate intermediate. A free radical competitive mechanism was found in the presence of alumina-supported heteropolyacids. (C) 2004 Elsevier B.V. All rights reserved.