The reactions of nucleoside 5’ triphosphates are among the most widespread (and therefore ancient) of the reactions of modern biochemistry. To study RNA enzymes that catalyze an important class of such reactions, we devised a new selection for RNAs that labilize their /3-y phosphates. We select RNAs that circularize in the presence of T4 RNA ligase, a reaction that requires a free 5’ a-phosphate and therefore indicates release of the /3r phosphates present in every transcript. Unexpectedly, free phosphates were found to react readily at the a-phosphate of such RNAs. This was emphasized by further selection for RNAs that self-link to the free triphosphate of UTP-agarose. The most prevalent ribozyme (40% of the final, doubly-selected pool) has been characterized. The uniform product of all such reactions is released pyrophosphate, and the RNA product has the attacking phosphate linked to the 5’ a-phosphate of the catalytic RNA. Thus, reaction with free GDP yields pyrophosphate plus a congener of the eukaryotic message cap (1): ppG + ppp(5’)GRNA ~ ppi + G(5’)ppp(5’)GRNA