A novel combination of a non-invasive imaging method with an oxygen sensitive fluorescent indicator was developed to investigate the biodegradation processes occurring at the fringe of a solute plume, where the supply of this electron acceptor is limited. A thin transparent porous matrix (156X120X3 mm) was made from quartz plates and quartz sand (212-300 micrometers). A substrate (acetate) was continuously injected into the matrix that had been enriched with acetate degrading bacteria. A degrading plume developed from the acetate source in the uniform flow field containing dissolved oxygen. Ruthenium (II)-dichlorotris(1,10-phenanthroline) (Ru(phen)3Cl2), a water soluble fluorescent dye, was used as an indicator of dissolved oxygen concentration. The fluorescence intensity is dependent on the concentration of oxygen because the dissolved oxygen acts as collisional quencher. The oxygen distribution within the matrix was interpreted from images recorded by a CCD camera. These two-dimensional experimental results show quantitatively how the oxygen concentrations decrease strongly at the narrow plume fringe and that oxygen was exhausted at the core of the plume. Separately, dispersivity was measured in a series of non-reactive transport experiments, and biodegradation parameters were evaluated by batch experiments. Two-dimensional numerical simulations with MT3D/RT3D used these parameters and the predicted oxygen distributions were compared with the experimental results. This measurement method provides a novel approach to investigate details of behavior of solute transport and biodegradation in porous media.