l-Rhamnose isomerase (l-RhI) catalyzes the reversible isomerization of l-rhamnose to l-rhamnulose. Pseudomonas stutzeril-RhI, with a broad substrate specificity, can catalyze not only the isomerization ofl-rhamnose, but also that betweend-allose andd-psicose. For the aldose–ketose isomer-ization by l-RhI, a metal-mediated hydride-shift mechanism has been proposed, but the catalytic mechanism is still not entirely understood.