AbstractThe stoichiometric stability constants (Kn; n = 1,2) of the lanthanon(III) complexes with isonitrosoacetone (I), isonitrosoacetophenone (II), isonitrosoacetylacetone (III), their respective oximes, ((IV), (V) and (VI), respectively) and dimethylglyoxime (VII), have been determined pH metrically in 1:1 v/v aqueous dioxan medium at 20° in presence of 0.10 M sodium perchlorate as the constant ionic medium using Calvin and Wilson's technique as given by Irving and Rossotti. The variations in the log K values with the change in the atomic number of the lanthanon(III) ion, change in the basicity of the ligands and the change from nitrogen donors to oxygen donors is discussed for the present set of the ligands. The lanthanon(III) ions prefer coordination through oxygen donors in comparison with the nitrogen donors