[en] Stability constants of the complexes of Ho(III), Dy(III), Tb(III), Sm(III), Nd(III), Pr(III) and La(III) with acenaphthenequinone monothiosemicarbazone have been determined potentiometrically in 75 per cent (v/v) aqueous dioxan medium at various ionic strengths and at different temperatures. The method of Bjerrum and Calvin, as modified by Irving and Rossotti, has been used to find out the values of n and pL. The stability constants have been calculated using an IBM 360 FORTRAN IV computer by the weighted least squares method. The order of the stability constants was found to be: Ho > Dy > Tb > Sm > Nd > Pr > La. Thermodynamic parameters such as ΔG, ΔH and ΔS have also been calculated. (author)