Periodate oxidation converted adenosine, guanosine, cytidine, and uridine into the corresponding dialdehydes. No free dialdehydes are present in solution, but several hydrated species occur (i.r. and n.m.r.). In the solid state, the dialdehydes are completely polymerized. The m.s. molecular peaks corresponding to the free daildehydes were of very low intensity. The c.d spectra of adenosine- and cytidinedialdehyde showed heavily diminished Cotton effects in comparison to the parent nucleosides. For uridine-dialdehyde, the signal intensity of the strongest transition was diminished by about one half, indicating a looser structure allowing free rotation of the base. By contrast, the c.d. signal of guanosine-dialdehyde was increased, indicating self-association of oligomers. The nucleoside-dialdehydes gave, with hydrazides, morpholine derivatives, which posses rigid, stable structures (c.d).