The Cocurrent Downflow Contactor (CDC) has been studied as a three phase reactor and in addition, chemically enhanced mass transfer studies have confirmed the very high gas hold-up values previously indicated by photographic methods (epsilon(g) = 0.5-0.6). Mass transfer measurements for the O-2/H2O system have shown that volumetric mass transfer coefficients (k(L) alpha) are in the range 0.25-0.3 s(-1) for unpacked and packed CDC reactors, while Co2+ ion-catalysed sulphite oxidation exhibited enhanced k(L) alpha values of 0.55 s(-1) with interfacial areas in the range of 1000-6000 m(2) m(-3). A model first order reaction, the palladium-catlysed hydrogenation of itaconic acid, was examined both in a small stirred batch reactor and in the CDC. Low degrees of mass transfer resistances were observed (both gas-liquid and liquid-solid) especially in the case of the CDC when used as a slurry and fixed bed reactor, with liquid-solid mass transfer resistances being the the range 1-10%. This was confirmed by energy of activation measurements in the range 30-45 kJ mol(-1). The CDC was used in slurry and fixed bed form for the respective hydrogenation of the triglycerides, rapeseed and soyabean oils. The reaction was predominantly surface reaction rate controlled with energies of activation in the range 47-58 kJ mol(-1), using palladium and nickel catlysts. Reaction selectivities were high, especially in respect of linolenate removal and the fixed bed CDC was slightly superior to the slurry reactor.