The tautomeric equilibrium of D-allose in aqueous solution is altered by the presence of some cations which form complexes with the α-pyranose and the α-furanose but not with the β-pyranose and the β-furanose forms. Determination of the proportions of the tautomeric forms by N.M.R. allows a comparison of the complexing ability of cations. Lanthanum, calcium, and strontium form the strongest complexes. The behaviour of D-lyxose, D-ribose, and D-glycero-D-gulo-heptose on complexing is described.