Department of Chemistry, Nagpur University, Nagpur-440 010 Manuscript received 2 January 1981, revised 17 April 1982, accepted 12 November 1982 The formation constants of the ternary complexes of neodymium(III), samarium(III), gadlonium(III) and holmium(III) with cyclohexanediaminetetraacetic acid (CyDTA) as primary ligand and dihydroxynaphthalene (DHN), dihydroxy��naphthalene-6-sulphonic acid (DHNSA) and catechol-3,5-disulphonic acid (CDSA) as secondary ligands have been investigated by potentiometric titration technique. The values of formation constants of 1 : 1 : 1 ternary chelates are reported at three different temperatures, and at a fixed ionic strength, ��=0.1 M (NaClO4).