Abstract1H NMR titration studies carried out at 300 or 360 MHz are described for RNase 1–120, RNase 1‐118 and five non‐covalent complexes of RNase 1‐118/RNase 111‐124, having L‐histidine, L‐homohistidine, L‐Nπ‐methylhistidine, L‐τ‐methyl‐histidine and 3‐(3‐pyrazolyl‐L‐alanine, respectively, as the active‐site amino acid in position 119. The pH titrations were performed both in the absence and in the presence of a RNase A inhibitor (3′‐CMP). From least‐squares analysis of the titration data, the most probable pKavalues and intrinsic chemical shifts of the histidine side‐chain in position 12 and 105 and of the amino acid residue in position 119 were obtained. On the basis of these results, the effects of replacing the amino acid residue in position