Sucrose isomerases from Protaminobacter rubrum, SmuA, and from Pseudomonas mesoacidophila MX-45, MutB, have been crystallized, and their three-dimensional structures solved. Determination of these crystal structures in their native states as well as in complex with substrate and substrate analogues have con- tributed to the visualization of a part of the double displacement reaction mechanism of this class of enzymes, and to the understanding of the specificity of the products. Comparative structural studies between the three- dimensional structures of trehalulose synthase, MutB, and the isomaltulose synthase, SmuA, have been conducted as well.