The Cocurrent Downflow Contactor (CDC) has been developed as a mass transfer and reactor device, with and without addition of tangential (swirl) flow, giving gas hold-up (E(g)) values of 0.5-0.75, interfacial areas in the range 1000-6000 m2 m-3 liquid and k(L)a values in the range of 0.15 - 1.55 s-1 for absorption using the O2/H2O system. It has been studied as a catalytic slurry reactor for the hydrogenation of (i) itaconic acid and (ii) triglycerides catalysed by Pd and Ni catalysts. The reactions were observed to be largely surface-reaction rate controlled, due to the very efficient mass transfer (k(L)a up to 11.75 s-1 under reaction conditions) and application of swirl flow-enhanced reaction rates. The CDC has recently been found to be capable of operating as a fixed bed reactor, thus eliminating a downstream catalyst separation problem (therefore more cost effective), and is superior in its mass transfer characteristics to other known devices. Scale-up can be undertaken without loss of performance efficiency.