The study of radionuclides speciation requires accurate evaluation of stability constants, which can be achieved by CE-ICPMS. We have previously described a method for 1:1 metal complexes stability constants determination. In this paper, we present its extension to the case of successive complexations and its application to uranyl-ox late and lanthanum-oxalate systems. Several significant steps are discussed: analytical conditions choice, mathematical treatment by non-linear regression, ligand concentration and ionic strength corrections. The following values were obtained: at infinite dilution, log(beta 1 degrees (UO(2)Oxa)) = 6.93 +/- 0.05, log(beta(2)degrees(UO2(Oxa)(2)(2-))) = 11.92 +/- 0.43 and log (beta(3)degrees (UO2(Oxa)(3)(4-))) = 15.11 +/- 0.12; log(beta(1)degrees(LaOxa(+))) = 5.90 +/- 0.07, log(beta(2)degrees(La(Oxa)(2)(-)) = 9.18 +/- 0.19 and log(beta(3)degrees(La(Oxa)(3)(3-) = 9.81 +/- 0.33. These values are in good agreement with the literature data, even though we suggest the existence of a new lanthanum-oxalate complex:)3 La(Oxa)(3)(3-). This study confirms the suitability of CE-ICPMS for complexation studies.