C2'-Nucleotide radicals have been proposed as key intermediates in direct strand break formation in RNA exposed to ionizing radiation. Uridin-2'-yl radical (1) was independently generated in single- and double-stranded RNA via photolysis of a ketone precursor. Direct stand breaks result from heterolytic cleavage of the adjacent C3'-carbon-oxygen bond. Trapping of 1 by O-2 or beta-mercaptoethanol (1 M) does not compete with strand scission, indicating that phosphate elimination is >10(6) s(-1). Uracil loss also does not compete with strand scission. When considered in conjunction with reports that nucleobase radicals produce 1, this chemistry explains why RNA is significantly more susceptible to strand scission by ionizing radiation (hydroxyl radical) than is DNA.