Application of heat to aqueous solutions of nucleoside dialdehydes (periodate-oxidized nucleosides) affords the corresponding alpha,beta-unsaturated aldehydes. The reaction was first discovered during studies with adenosine deaminase and was initially investigated enzymatically until the nature of the chemical transformation was determined. A UV peak at 230-250 n, characteristic of the alpha,beta-unsaturated aldehyde group, was obtained by difference spectroscopy and affords a more practical means to study the reaction. Adenosine dialdehyde, obtained by periodate oxidation of adenosine, afforded the same product upon heating as obtained by periodate oxidation of 9-(5-deoxy-beta-D-erythro-ent-4-enofuranosyl)adenine. Further proof of identity was obtained by reduction of these compounds with sodium borohydride and comparison of the dialchols obtained to each other and to a known unsaturated dialcohol. Heating of nucleoside dialdehydes at any time is not recommended. The exact composition of nucleoside dialdehydes used in previous and current biological studies is open to question.