2,5-furandicarboxylic acid (FDCA) is a potential non-phthalate based bio-renewable substitute for terephthalic acid-based plastics. Herein, we present an investigation of the oxidation rate of 5-hydroxymethylfurfural (HMF) to FDCA in acetic acid medium using Co/Mn/Br catalyst. Transient concentration profiles of the reactant (HMF), intermediates [2,5-diformylfuran (DFF), 5-formyl-2-furancarboxylic acid (FFCA)], and the desired product (FDCA) were obtained for this relatively fast reaction in a stirred semi-batch reactor using rapid in-line sampling. Comparison of the effective rate constants for the series oxidation steps with predicted gas-liquid mass transfer coefficients reveals that except for the FFCA FDCA step, the first two oxidation steps are subject to gas-liquid mass transfer limitations even at high stirrer speeds. Novel reactor configurations, such as a reactor in which the reaction mixture is dispersed as fine droplets into a gas phase containing oxygen, are required to overcome oxygen starvation in the liquid phase and further intensify FDCA production. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 162-171, 2017