A liquid core waveguide membrane microreactor is a novel concept that allows in situ spectroscopy to be carried out in an optical microreactor, while enabling a very high gas-to-liquid mass transfer. The potential of this reactor concept to derive kinetics in gas/liquid reactions is demonstrated using the heteropoly acids catalyzed oxidation of biomass to formic acid (the 'OxFA process'). For vanadium-substituted, Keggin-type heteropoly acids, the kinetics of the reduction and reoxidation step of the catalyst cycle was deduced using in situ UV/Vis spectroscopy. Reduction was studied from 40 to 80 degrees C: a strong dependency of the reduction kinetics was deduced. In combination with additional cyclic voltammetry and EPR characterization, it can be assumed that two active centers promote this reaction, while the existence and proportions of both types depends on the degree of vanadium substitution. The rapid mass transfer allowed transient response experiments to be carried out for the reoxidation step, which was studied up to 150 degrees C. Switching from anaerobic to aerobic conditions, an even stronger dependency of the reoxidation kinetics on the degree of vanadium substitution was revealed.