The plant root and plant-produced compounds or root exudates provide a local environment rich in nutrients and enzymes for enhanced microbial population and activity in the soil zone near the root surface - the rhizosphere. Mathematical modeling suggests that the spatial extent of the rhizosphere and degradation rate within the rhizosphere is an important parameter in the degradation of immobile constituents in petroleum-contaminated soil by phytoremediation based on rhizodegradation. In this study the rhizosphere extent is calculated from a digitized high-resolution in-situ (non-destructively obtained) image of the root zone during phytoremediation of a pyrene or phenanthrene thin film. Initial estimates of the extent of the rhizosphere surrounding Bermuda grass suggest that it is only a few hundred microns in extent; on the order of the root diameter. Further, quantitative study of the fluorescent thin films has allowed in-situ calculation of degradation rates in rhizosphere zones surrounding fine root structures.