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-oxalate 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 (UO(2)(Oxa)(2) (2-)))=11.92+/-0.43 and log(beta(3) degrees (UO(2)(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: La(Oxa)(3) (3-). This study confirms the suitability of CE-ICPMS for complexation studies.