Erlose [O-beta-D-fructofuranosyl-(1-->2)-O-alpha-D-glucopyranosyl-(1-->4)- alpha-D-glucopyranoside] monohydrate, C18H32O16.H2O, M(r) = 522.45, is orthorhombic, P2(1)2(1)2(1) with a = 30.748 (3), b = 8.757 (1), c = 8.270 (1) A, and Z = 4. The structure was solved by direct methods, and refined to R = 0.048 for 1909 observed reflections. The torsion angles about the (1-->2) and (1-->4) glycosidic bonds are similar to those observed in other sucrose- and maltose-like oligosaccharides. The maltose moiety has an O-3'-H...O-2 intramolecular hydrogen-bond, but the sucrose moiety has no intramolecular hydrogen bonds. The hydrogen bonding in the crystal includes infinite and finite chains crosslinked by the water molecule.