In general, pK a shifts for acids, such as thiols, carboxylic acids, phenols and phosphoric acid monoanions, are considered to occur under homogeneous conditions and change with dielectric constant. Theoretical calculations have been performed for homogeneous environments [1][2][3][4], and some improved theoretical studies have considered anisotropic and heterogeneous environments [4][5][6][7][8] to deal with the particularly large shift for thiols, phenols and carboxylic acids located inside proteins. This paper discusses pK a shifts for acids having neighbouring amide NHs.A relationship between the pK a shift and the formation constant is observed for metal complexes in hydrophobic environments. The influence of intramolecular NHÁ Á ÁX hydrogen bonds on the metal-X bond character is also observed for metal complexes. The presence of NHÁ Á ÁS hydrogen bonds in iron-sulfur proteins observed by crystallographic studies has been suggested by Adman and coworkers [9]. Various synthetic metal-thiolate complexes having chelating Cys-containing oligopeptides and synthetic simple intramolecularly NHÁ Á ÁS hydrogen-bonded thiolate ligands exist. NHÁ Á ÁS hydrogen bonds in metal-thiolate complexes play an important role in the following functions.Intramolecular hydrogen bonds contribute to the following: increase in the stabilization constant, e.g. Fe(II), Zn(II), Cd(II), Hg(II) and Ca(II) complexes; positive shift of the redox potential, e.g. Fe(II), Fe(III), Mo(IV) and W(IV) complexes; and increase in air stabilization, e.g. Fe(II) and Fe(III) complexes. In addition, the NHÁ Á ÁS hydrogen bond increases the formation constant for metal-sulfur complexes formed by substitution reaction between non-hydrogen-bonded and hydrogen-bonded thiolates [10]. The large formation constant is due not only to the pK a -lowering shift of thiol but also to a pp-dp interaction by the NHÁ Á ÁS hydrogen bond.