Department of Chemistry, Malaviya Regional Engineering College, Jaipur-302 004 Manuscript received 12 June 1980, revised 17 September 1981, accepted 5 November 1981 The complexation of La+3, Ce+3,Pr+3 and Sm+3 ions with N,N-\({bis}\)(2-hydroxyethyl) glycine has been studied by potentiometric titration technique in aqueous medium. All these metal ions form 1 :1 and 1 : 2 complexes in the \({p}\)H range 4.0-8.0 with considerable overlapping. The log K values are determined at 20˚, 30˚ and 40˚ at ionic strength \(\mu\) =0.1M (NaClO4) by applying Calvin and Melchior's extension of Bjerrum method. The stability constants follow the order : Sm(III)>Pr(III)> Ce(III)> La(III).