Oxidation of 5-hydroxymethylfurfural (HMF) using air or pure oxygen was performed in polytetrafluoroethylene capillary microreactors under gas-liquid slug flow, with Co/Mn/Br as the homogeneous catalyst in the acetic acid solvent. The temperature was varied from 90 to 165 • C at a pressure of 1 or 5 bar. At atmospheric pressure conditions (and 90 • C), acetaldehyde was further added as a co-oxidant to accelerate the reaction. At 150 • C, 5 bar oxygen and a residence time of 2.73 min, an HMF conversion of 99.2% was obtained, with the yields of 2,5diformylfuran (DFF), 5-formylfurancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA) being 22.9%, 46.7%, and 23.8%, respectively. By operation under wetted slug flows and elevated partial oxygen pressures, mass transfer limitations and oxygen depletion in the microreactor could be eliminated. This allowed to run the microreactor under kinetically controlled conditions, where both the HMF consumption and DFF formation were found zero order in partial oxygen pressure and roughly first order in HMF. The total selectivity towards DFF/ FFCA/FDCA was ca. 40% at low partial oxygen pressures due to the dominant occurrence of side reactions. By using pure oxygen at 5 bar the total selectivity was improved to 60-94%. The space time yields of DFF and FFCA in the microreactor exceeded those obtained in conventional (semi-)batch reactors at slightly elevated temperatures and pressures, due to the superior mass transfer and higher initial HMF concentrations in the microreactor. For highly efficient FDCA synthesis, more dedicated microreactor operations are needed to tackle its precipitation.