Three novel substrates-D-arabinose, L-ribose, and D-lyxose-are described for the industrially important xylose isomerase (glucose isomerase). Furthermore, it was shown that contrary to previous studies, two different reaction products are formed when pentose sugars are used as substrates. In extended reactions an apparent equilibrium is formed between both D- and L-forms of arabinose, ribulose, and ribose as well as D-xylose, D-lyxose, and D-xylulose. The new reactions are discussed in the light df previously described reaction mechanisms for this enzyme.