The title compound, C(13)H(24)O(11).4H(2)O, (I), crystallized from water, has an internal glycosidic linkage conformation having phi' (O5(Gal)-C1(Gal)-O1(Gal)-C4(All)) = -96.40 (12) degrees and psi' (C1(Gal)-O1(Gal)-C4(All)-C5(All)) = -160.93 (10) degrees , where ring-atom numbering conforms to the convention in which C1 denotes the anomeric C atom, C5 the ring atom bearing the exocyclic hydroxymethyl group, and C6 the exocyclic hydroxymethyl (CH(2)OH) C atom in the betaGalp and betaAllp residues. Internal linkage conformations in the crystal structures of the structurally related disaccharides methyl beta-lactoside [methyl beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranoside] methanol solvate [Stenutz, Shang & Serianni (1999). Acta Cryst. C55, 1719-1721], (II), and methyl beta-cellobioside [methyl beta-D-glucopyranosyl-(1-->4)-beta-D-glucopyranoside] methanol solvate [Ham & Williams (1970). Acta Cryst. B26, 1373-1383], (III), are characterized by phi' = -88.4 (2) degrees and psi' = -161.3 (2) degrees , and phi' = -91.1 degrees and psi' = -160.7 degrees , respectively. Inter-residue hydrogen bonding is observed between O3(Glc) and O5(Gal/Glc) in the crystal structures of (II) and (III), suggesting a role in determining their preferred linkage conformations. An analogous inter-residue hydrogen bond does not exist in (I) due to the axial orientation of O3(All), yet its internal linkage conformation is very similar to those of (II) and (III).