The hydrogenation of 4-nitrobenzoic acid to 4-aminobenzoic acid was studied in a circular capillary, wash coated with a Pd on alumina catalyst, operated in vertical two phase down flow. The influence of multiphase flow pattern (slug (Taylor) flow, annular, bubble and chum) upon the observed reaction rate was investigated; in slug flow the reaction rate was a function of the liquid slug and gas bubble lengths. Mass transfer models for the different flow patterns are proposed based on previous mass transfer coefficient models. (c) 2006 Elsevier Ltd. All rights reserved.