The proportion of the pyranose and furanose forms in aqueous solutions of deoxyaldoses has been determined by p.m.r. spectroscopy. Removal of a hydroxyl group from C2 or C3 of an aldose causes an increase in the furanose content, particularly if the group was cis to another group. The strongest unfavourable interaction in the furanose ring appears to be the one between the side-chain and a neighbouring cis hydroxyl group.