Aims: The formation constants of histidine complexes of Fe (III), Co (II), Ni (II), Cu(II), Zn (II) and Pb (II) at temperatures of 25, 30, 37, 45°C were determined potentiometrically, utilizing modified Bjerrum's method. Potentiometric titrations were carried out in 30% and 70% (v/v); 50% and 50% (v/v) water-dioxane mixtures. Ionic strength of medium was retained at 0.10 M by sodium nitrate. The first and second formation constants were obtained by plotting corrected pH versus concentration of standardized NaOH solution after each addition. Results: The trend for increasing stability in both aqueous solution and in dioxane-water mixtures is as follows: k f pb(II)-His < k f Co (II)-His < k f Zn (II)-His < k f Ni(II)-His < k f Cu(II)-His < k f Fe(III)-His The effects of dioxane and temperature on the formation constants of histidine complexes were examined. Our findings showed that the formation constants of the complexes increased as the