The formation constants of binary ML and ML2 and ternary MAL complexes where M = Co(II), Ni(II), Cu(II), or Zn(II), L = O, O- donor isatin or 2,3-indolinedione, and A = (i) N, O- donor glycine, alanine, or valine, (ii) N, N donor ethylenediamine or 2,2’-bipyridyl, or (iii) O-, O- donor oxalic acid or catechol have been determined from solution equilibrium studies by potentiometric pH measurements at 30 +/- 1-degree-C and 0.1 mol.dm-3 (KNO3) ionic strength in 5 vol % ethanol-water. The stepwise formation constants of these complexes followed the sequence log K(ML)M > log K(MAL)MA > log K(ML)2ML. The relative stabilities of the ternary complexes have been characterized in terms of the statistical parameter DELTA log K. The results showed an increasing order of stability with respect to the nature of donor atoms on A as O-, O- < N, O- < N, N. This sequence has been attributed to various astatistical factors. The stabilities of all the complexes also followed the (natural) order Co(II) < Ni(II) < Cu(II) > Zn(II).