One aspect of the invention relates to industrial bioconversion of the xylose portion of biomass materials into fuels and chemicals. Another aspect of the invention relates to industrial bioconversion of the xylan portion of biomass materials into fuels and chemicals. In one embodiment, the invention is directed to the bacterium Clostridium thermocellum, a highly cellulolytic organism that has much potential as a biocatalyst in a consolidated bioprocess configuration. In some embodiments, the invention is a genetic modification that confers the ability to ferment xylose to C. thermocellum and the strains created with this modification. In some embodiments, the genetic modification is composed of two genes contained in an operon from T. saccharolyticum. The genes express proteins with xylose isomerase (XI) and xylulokinase activites (XK). In other embodiments, the invention relates to a recombinant Clostridium thermocellum host cell capable of fermenting xylan.