The reaction of chloroacetaldehyde with adenosine and cytidine produces a fluorescent product in each case. These products are easily distinguishable spectroscopically by their fluorescence emission maxima. Since the reaction is carried out in aqueous media under mild conditions of pH and temperature, it should prove extremely useful in nucleic acid chemistry. For example, reaction occurs between chloroacetaldehyde and E. coli tRNA, and the reaction can be followed by monitoring the increase in the fluorescence emission. Fluorescence lifetimes observed for the adenosine and cytidine products are close to 20 and 7 nsec, respectively. © 1972 Academic Press, Inc.