AbstractThe acidity constants of the six thienyl‐ and the three phenyl‐pyridines, and the stability constants of the corresponding Cu2+1:1 complexes were determined in aqueous solution (I= 0.1, NaCIO4; 25°). The basicity of the pyridyl group is increased by a shift of a thienyl or phenyl group in the order 2<3<4 and the influence of the 2′‐thienyl group is greater than that of the 3′‐thienyl group. The stability of the Cu2+1:1 complexes formed with 3‐ and 4‐substituted pyridines is only dependent on their basicity, as is commonly observed for structurally related ligands and their complexes. Due to steric hindrance the complexes of the 2‐substituted derivatives are less stable by about 1.5 log units. There is no evidence that the aromatic bonded sulfur of 2‐(2′‐thienyl)‐pyridine participates in complex formation with Cu2+, in oppositi