Thermometric measurement of vapor pressure equilibria in reaction mixtures containing borate ion and cis-3,4-dihydroxytetrahydrofuran (I) or D-glucose 5,6-carbonate (II) indicate that complexing involves at least three different equilibria. Borate complexing by I is characterized by a gross change in n.m.r. spectral characteristics, most striking being a strong overall decoupling effect. Alterations in the spectrum of II and of 5-O-methyl D-glucose in the presence of borate, when compared with those of D-glucose, suggest that complex formation by the latter sugar proceeds with a pyranose-to-furanose interconversion. The O—C—C—O dihedral angle, within the range 0° to about 40°, does not appear to be a factor determining the stability of borate complexes. Crystalline spirane-type complexes of I, II, and of D-threose have been prepared, illustrating the usefulness of borate complexing for the isolation of some furanose sugars and derivatives.Solutions of alkali tetraborates are found by thermometric vapor pressure measurements to behave as equimolecular mixtures of alkali borate and boric acid.