[en] The thermodynamic parameters for the complexation of four solochrome dyes viz. Solo yellow 2 GS, Solo Black WDFA, Solo 6 BN and Solo Fast Navy 2 RS, with lanthanum, cerium, thorium and uranium have been determined potentiometrically, at 280 and 440 and at an ionic strength of μ=0.1, in aqueous solutions. The step wise values of ΔG, ΔH and ΔS have also been evaluated. (author)