Abstract: Phytoremediation of organics is presumed to be based on the stimulation of microorganisms in the rhizosphere, resulting in a higher microbial population and activity and an altered community composition. Substrate utilization patterns of BIOLOG Eco PlatesTM, consisting of 31 carbon sources, were used to determine the effect of the tropical grass Brachiaria brizantha on microbial carbon source utilization patterns in heavy crude oil contaminated soil and to find a possible correlation with enhanced crude oil degradation found in previous experiments. Counts of numbers of carbon sources used only resulted in differences between contaminated and uncontaminated soil while Principal Component Analysis (PCA) showed different carbon source utilization patterns between rhizosphere soil and non-rhizosphere or unplanted soil. Similar to its influence on oil degradation, B. brizantha had a significant effect on microbial community patterns after 14 weeks but only a minor effect after 30 weeks. In particular, D, L-α-glycerol phosphate was more used by microbial communities of the rhizosphere, pointing to a possible key role of phosphorus in the highly nutrient deficient savannah soils of Venezuela and its mobilization in the rhizosphere. Missing incisiveness of the results can be related to the fact that fungi might play a central role in phytoremediation of petroleum hydrocarbons in the tropics but are not detected by the BIOLOG assay.