Department of Chemistry, Vidarbha Mahavidyalaya, Amravati-444 604 Manuscript received 21 September 1981, revised 5 September 1983, accepted 19 January 1985 The interaction of FeIII with 2'-hydroxy-4-methoxy-5'-methylchalcone (1), 2'- hydroxy-4-methoxychalcone (2), 2'-hydroxy-3'-bromo-4-methoxy-5'-methylchalcone (3) and 2'-hydroxy-4-methoxy-5',6 -benzochalcone (4) has been investigated potentiometri­cally in 70% dioxane-water mixture at 0.1 M ionic strength. The study of FeIII com­plexes with 2'-hydroxy-4-methoxy-5'-methylchalcone has also been done at 0.032, 0.052, 0.072, 0.092 and 0.112 M ionic strength. It is observed that ferric ion forms 1 : 1, 1 : 2 and 1:3 complexes with all the ligands. Proton-ligand and metal-ligand stability constants have been determined by Bjerrum method as adopted by Calvin and Wilson. The data of pK and log K values obtained from this technique was used to (i) verify the relation log K=a pK+b, (ii) calculate the thermodynamic dissociation constants at zero ionic strength, and (iii) find the closest distance of approach between metal ion and ligand.