The present study includes important findings relating to the number of donor atoms, species of ligands, and stabilities of complexes. Stabilities of complexes between Cu(II) ion and NO-, NS-, ONS-, and ONO-type Schiff bases were compared. Acid-base properties of the Schiff bases were explained at 25 +/- 0.1 degrees C and ionic strength (I) of 0.1 M supported by NaCl. The Hyperquad computer program was used for calculation of dissociation and stability constants. The overall stability constants of their Cu(II) complexes were calculated and the various formed complexes between the Schiff bases with Cu(II) ion formulated as CuL2, CuHL2, CuH2L2, and CuH-1L2 (Cu(OH)L-2). The complexes of ONS- and ONO-type tridentate ligands were more stable than those of NO- and NS-type bidentate ligands.