By means of various physical techniques D-arabinose oxime has been shown to exist in the anti-(Z-) acyclic form in the solid state and to isomerise to a mixture of anti-(Z)-(20 %) and syn-(E-)(80 %) forms in aqueous solution. D-glucose oxime exists in the cyclic β-pyranose form in the solid state and isomerises to a mixture of β-pyranose (23 %), α-pyranose (7 %), anti-(Z-)(13.5 %), and syn-(E-)(56.5 %) forms in aqueous solution. Conformational analysis of one cyclic (β-D-gluco-) and of one acyclic [(E)-D-arabino-] form has been carried out by 1H n.m.r. spectroscopy.