Abstract: The binary and ternary stability constants of DL-α-Alaninc with Cu(Ⅱ),Zn(Ⅱ),Ni(Ⅱ) and Cd(Ⅱ) were determined by pHpolcntiometric titration at 37±0.1℃ and ionic strength 150mM/dm³.Potentiomeinc data were treated with ZPLOT,MAGEC,MINIQUADand ESTAprograms.The compositions of complex species were ascertained in the complex equilibriums by selection of different chemical models.The lgKand lgXvalues indicate that the complexes with mixed li-gands were obviously more stable than the simple binary systems especially in the zinc-alaninate-histidinale-proton system.Distribution curves of the complex species show the continuity of stepwise cotnplexing in the formation of complexes.At physiological pHregion these complexes,except cadium system,existed mainly in the form of neutral complexes which have biological significance in the transfer of metal ons acrossing the membrane.