A general acid-base catalytic mechanism for ribonuclease A and other ribonucleases was previously widely accepted. However, an alternative to this mechanism was recently reintroduced in which attack by the 2’ hydroxyl group is facilitated by protonation of a nonbridging phosphoryl oxygen atom, instead of the leaving group oxygen atom, to give a triester-like mechanism of catalysis. Literature values for rate effects upon substitution of nonbridging phosphoryl oxygen atoms by sulfur (i.e., "thio-effects") for enzymatic and nonenzymatic reactions of RNA and for nonenzymatic reactions of other phosphate diesters and triesters are compared herein. The thio-effects observed in the RNase A-catalyzed reactions are consistent with predictions based on the classical general acid-base catalyzed mechanism but are inconsistent with predictions based on the triester-like mechanism. Thus, the results of this analysis support the classical general acid-base mechanism rather than the triester-like mechanism. In addition, the results suggest that short, strong hydrogen bonds do not contribute substantially to RNase A catalysis.