Tris (2-amino-2-hydroxymethylpropane-1,3-diol) reacts with aldehydes of low molecular weight in aqueous solutions to give products whose NMR spectral properties are inconsistent with the formation of the expected Schiff base. High-field 1 H and 1 3 C NMR spectra of a solution of Tris and glyceraldehyde 3-phosphate indicated initial formation of 2-(β-phosphoglycolyl)-4,4-bis(hydroxymethyl)1,3-oxazolidine, which was then converted to condensation products of Tris and methylglyoxal, accompanied by deuterium exchange when the solvent was 2 H 2 O. Acetaldehyde and benzaldehyde were used as model substrates which yielded more stable oxazolidines in their reactions with Tris.