The rate at which dinucleoside phosphates are cleaved by RNases is supposed to be determined by the mole fraction of enzyme-substrate complexes in which the phosphodiester moiety of a dinucleoside phosphate has a highly reactive conformation. The mole fraction of such complexes for a particular RNase depends on the nature of a nucleoside at the O5'-end of the phosphodiester bond. Experimental data are presented to support this hypothesis.