Three different alternatives are applied for the determination of the stability constants of zirconium and hafnium complexes with 2-methyl lactic acid using cation exchange. Calculations based on the exchange equilibria of the highly charged metal cations and the positively charged ligand complexes rather than that of the hydrolysed complex species are worked out according to SCHUBERT and FRONAEUS, respectively. The third alternative adopted in this work, is principally based on the consideration of all complexed species produced by hydrolyses in the aqueous phase. The B-values obtained by the different methods are intercorrelated and discussed.