Measurements of mass transfer in a highly viscous pseudoplastic broth, which is typical to Xanthomonas campestris fermentations, are difficult to obtain by conventional methods and little data is available. A novel research method that uses bioluminescence for mass transfer studies has been developed. A plasmid carrying the luminescence operon of marine luminous bacteria is introduced into an industrial bacteria, X. campestris. Besides producing the polysaccharide xanthangum, the bioluminescent X. campestris emits measurable light. Monitoring the luminescence is a simple, noncontaminating nondestructive and very sensitive indicator of the metabolic activity of the culture during fermentation. Energy drain due to bioluminescence is very low; growth rate and polysaccharide production rate are close to those of the wild‐type strain. Oxygen and substrate mass transfer are determined by inducing step or periodic fluctuations in their concentration and measuring the resultant luminescence response. Oxygen mass transfer coefficients show linear dependence on Reynolds number and an exponential dependence on the average shear rate. Viscosity effect is small at high viscosities but increases rapidly below 10 Pa‐s. The influence of oxygen uptake rate is studied. Mass transfer of the limiting component (ammonium ions) is analyzed under pulsating feed conditions. The luminescence declines, following a feed pulse, due to energy investment in active transport of ammonium ions through the cell membrane, it regenerates then to its baseline. The relation between mass transfer and luminescence fluctuation is elucidated.