Department of Chemistry, Bipin Bihari College, Jhans1-284 001 Manuscript received 1 February 1977, revised 12 June 1978, accepted 1 August 1978 The stability constant (log k) of La(III), Ce(III), Pr(III), Nd(III) and Gd(III) with phloracetophenone (2, 4, 6-trihydroxy-acetophenone) have been evaluated by Irving-­Rossotti technique at 25±0.1° in 50% dixoane-H2O system at an ionic strength of 0.1M NaCIO4. The stoichiometric ratio of metal-ligand (1 : 1) have been studied by \(p\)H-metric titration method. The order of stability constants follow the well established order, i.e.,— La(III) <Ce(III) ≤Pr (III) ≤Nd (III) ≤ Gd(III) The values of stability constant (log k) and change in free energy (∆G) in K cal/mole for La(Ill), Nd(III) and Gd(III) are 3.19, 3.39, 3.31, 3.71, 3.69 and 4.36, – 4.61, – 4.51, 5.06, – 5.04 respectively.