A process for performing a chemical reaction between a plurality of chemical reactants in the presence of a catalyst, with at least a first reactant in a liquid phase and at least a second reactant in a gas phase, the first and second reactants mixing at least by interdiffusion. The process comprises supplying the reactants to a passage defined by a first surface and a second surface and optionally moving at least one of the first surface and second surface relative to each other. The reactants their mixture or reaction products form respective boundary layers against the first and second surfaces and the radial spacing between the first surface and second surface is equal to or less than the back-to-back radial thicknesses of the boundary layers.