The proton ligand stability constants of 3 (4?-chlorophenyl) - 4- benzoyl -5 (2-hydroxy phenyl) pyrazole (L) and metal ligand stability constants of Pr (III), Nd (III) and Sm (III) complexes have been investigated at 0.02, 0.04, 0.06, 0.08 and 0.1 M ionic strength in dioxane-water mixture, pH metrically. From the values of pK and logK, the values of thermodynamic constants at zero ionic strength can be calculated. The value of pK and logK are found to be decreased with increasing ionic strengths of 1M KNO3 solution