A substrate specificity study of the recombinant D-ribose-5-phosphate isomerase (RpiB) from Clostridium thermocellum was performed. Among all aldopentoses and aldohexoses, the RpiB enzyme displayed activity with L-talose, D-ribose, D-allose, L-allose, L-ribose, and D-talose in decreasing order. The products released were L-tagatose, D-ribulose, D-psicose, L-psicose, L-ribulose, and D-tagatose, respectively. The enzyme showed specificity for aldose substrates possessing hydroxyl groups oriented in the same direction at the C2, C3, and C4 positions. Molecular modeling of the enzyme suggests that the novel substrate specificity may be explained by substrate interactions with residues Tyr42, His98, and His9, which interact with the hydroxyl groups of C2, C3, and C4, respectively, oriented in the same direction. L-Talose and D-ribulose exhibited the highest activity among the aldoses and ketoses, respectively. Ribose 5-phosphate isomerase catalyzed the conversion of L-talose to L-tagatose with an 89% conversion yield after approximately 90 min, while D-ribulose was converted to D-ribose with a 38% conversion yield.