The complex formation between Th(III), Sm(III), Nd(III) and Pr(III) Metal Ions And 2-Hydroxy-4-(4’- Aminophenyl)-6-(2’-Hydroxy-5’-Bromophenyl)Pyrimidine[H4AHBP](L1) and 2- Hydroxy -4-(4’-Chlorophenyl)-6- (2’-Hydroxy-5’-Bromophenyl)Pyrimidine[H4CHBP](L2) have been studied at 0.1 M Ionic Strength(30±0.1) 0C in 70 % Dioxane-water mixture by Bjerrum method as adopted by Calvin and Wilson. It is observed that Th(III), Sm(III), Nd(III) and Pr(III) Metal ions form 1:1 and 1:2 complexes with ligands (L1 & L2). The data obtained were used to estimate and compare the values of proton-ligand stability constant (pK) and metal-ligand stability constants (log k). From estimated data (pK and log k), the effects of substituents were studied.