Bovine pancreatic ribonuclease A is an enzyme that catalyses the depolymerization of RNA. This process involves the interaction of the enzyme with the polymeric substrate in the active site and its correct alignment on the surface of the enzyme through multiple binding subsites that essentially recognize the negatively charged phosphate groups of the substrate. The enzyme shows a strong specificity for pyrimidine bases at the 3'-position of the phosphodiester bond that is cleaved and a preference for purine bases at the 5'-position and, probably, for guanine at the next position. On the other hand, the enzyme shows a clear preference for polynucleotide substrates over oligonucleotides. In this review the contributions to the catalytic mechanism of some amino-acid residues that are located at non catalytic binding subsites are analysed.