The bidentate Schiff base ligand 4-chloro-2-[1-(4-phenylphenyl) ethylideneamino] phenol and its mononuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes have been synthesized. Ligand and metal complexes were characterized by elemental analyses, magnetic susceptibility, molar conductivity, H-1-and C-13-NMR, the Fourier transform infrared, UV-Vis, inductively coupled plasma optical emission spectrometry, and thermogravimetric-differential thermogravimetric studies. The results suggest that the mononuclear complexes have a metal to ligand mole ratio of 1: 2 and the metal(II) ions are coordinated with the phenolic oxygen and imine nitrogen atoms. Octahedral structures are proposed for the complexes of the Schiff base ligand. Furthermore, the complexes were checked for their efficiency to decolorize the dye methylene blue. In our experiments we found that metal(II) complexes had an acceptable decolorization efficiency against the dye methylene blue.