AbstractThe complexation of La3+, Ce3+, Pr3+and Sm3+ions with isopropylmercaptan has been studied by potentiometric and conductometric titration techniques in aqueous – 10% ethanolic (V/V) medium. Title metal ions form 1:1, 1:2 and 1:3 complexes in the pH range 6.0–8.0 with considerable overlapping. Their log Kstab.values are determined at 15°, 25° and 35°C at ionic strength μ = 0.1 M (NaClO4) by applying Calvin and Melchior's extension of Bjerrum method. The values of thermodynamic parameters ΔG, ΔH and ΔS have been calculated at 25°C with the help of an isobar and Gibb's Helmholtz equation. The trend in the stability constant values of the metal‐complexes has been found to be\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm Sm}\left({{\rm III}} \right) > {\rm Pr}\left({{\rm III}} \right) > {\rm Ce}\left({{\rm III}} \right) > {\rm La}\left({{\rm III}} \right) $$\