Department of Chemistry, Ramnarain Ruia College, Bombay-100 019 Manuscript received 2 January 1979, revised 24 January 1980, accepted 26 July 1980 "Potentiometric studies have been carried out on metal complexes of Y3+, La3+, Pr3+, Nd3+, Gd3+ and Dy3+ with 2-hydroxy-1-naphthalidene-2'-methyl-4'-nitroaniline. The dissociation constants (pK1+pK2) of the reagent and the formation constants of its metal complexes have been determined by Bjerrum method at 25����0.1�� and at an ionic strength 0.1M in 75 : 25 percent (v/v) dioxan-water medium. The validity of Born's relation was examined by studying the plot of Z2/r vs log k values of rare earth complexes of the ligand.