Natural cellulolytic environments offer a plethora of novel organisms and metabolic capabilities for bioenergy production, but their potential remains to be properly evaluated by comparative techniques. Enriched microbial communities with high cellulolytic activity have been previously described, with varying complexity and community composition based on 16S rRNA and glycosyl hydrolase family 48 gene analyses. These consortia were either dominated by Clostridium clariflavum or Clostridium straminisolvens –like strains. Some of these consortia included organisms able to degrade both cellulose and xylan at similar rates. To fully understand differences in their ability to utilize cellulose and xylan, established cellulolytic consortia have been grown in batch cellulose and xylan fermentations with on-line monitoring of changes in pH and changes in CO₂ and H₂ production. In addition, we have incorporated total organic carbon/nitrogen measurements and HPLC analysis of organic acids and residual sugars for the duration of each fermentation. We have observed distinct differences between these consortia in their ability to utilize both cellulose and xylan, which seems to be directly linked to their community composition. Additionally, end-product ratios varied depending on the dominant community member, with formate becoming more prevalent in communities dominated by C. clarivlavum strains, whereas acetate and ethanol were more prevalent in communities dominated by C. straminisolvens strains. In all, detailed metabolic and molecular characterizations are provided for these communities by a combination of techniques, resulting in detailed comparisons between consortia in their ability to utilize xylan and cellulose.