The stability constants of the complexes of yttrium and the lanthanide ions (except Pm) with sulfate were determined using a competitive potentiometric method at μ = 2·00 M (NaClO4), pH = 3·0 and t = 25·0°C. A solvent extraction technique was used to measure stability constants of the sulfatecomplexes of Ce(III), Pm(III), Eu(III), Am(III), Cm(III) and Cf(III) with the same set of conditions. With